Adhesives were utilized in a sophisticated manner even in ancient times. Recent years have seen the rapid development of adhesive bonding as an economic and effective method for the fabrication of components and assemblies. The great many types of adhesives are currently in use and there is no adequate single system of classification for all products. The adhesives industry has generally employed classifications based on end use, such as metal to metal adhesives, wood adhesives, general purpose adhesives, paper and packaging adhesives etc. An adhesive or formulation is generally a mixture of several materials. The extent of mixture and the ratio usually depend upon the properties desired in the final bonded joint. The basic materials may be defined as those substances, which provide the necessary adhesive and binding properties. The type of adhesive material is easier to define and usually falls into three categories; thermosetting resins, thermoplastic resins and elastomeric resins. A thermosetting system, 100 percent reactive when in a pure state, the epoxies are very desirable and more widely used than any other chemical type. Epoxy is one of the newer types and has penetrated more fields of manufacturing operations in a shorter space of time than any of its predecessors. The many catalysts used with epoxies produce systems of variable properties. The most common are the aromatic amines and cyclic anhydrides. The phenolics or phenol formaldehyde resins are formed by the condensation reaction of phenol and formaldehyde. The phenolic resins have been used extensively in the lamination of plywood and in filament wound structures. There are two basic classes of phenolic resins resoles and novalacs, and both begin as phenol alcohols. When combined or alloyed with other adhesive systems, they become excellent structural adhesives and are widely used in this manner throughout the aerospace industry. The vinyl polymers do not stand alone as a structural adhesive, but hundreds of adhesives are formulated by the use of this class of polymer. The vinyls are important to adhesive bonding not only from the adhesive standpoint, but because the films derived from these substances are widely used as vacuum bags, slip sheets, etc. The more widely used ones are polyvinyl chloride, polyvinyl alcohol, and polyvinyl fluoride. There are numerous kinds of adhesives used in different industries; polyvinyl acetate wood adhesives, aminoresin wood adhesives, phenolic resin wood adhesives, cynoacrylate adhesives, hot melt adhesives, water based adhesives etc. The market for adhesives is comprised of thousands of end uses. The realm of market applications expands as new end uses keep developing, driven by the need for new and innovative attachment solutions. When looking at the total market, adhesives account for about 75% of the volume consumed.
This book basically deals with adhesive properties and general characteristics, adhesive materials and properties, adhesives types, thermoplastic adhesives, thermosetting adhesives, rubber resin blends, properties of basic adhesives types, acrylics acrylic acid diesters, allyl diglycol, carbonate, animal glues, blood albumen, butadiene styrene rubbers, butyl rubber and polyisobutylene casein, cellulose derivatives, cellulose acetate, acetate butyrate cellulose, caprate cellulose, nitrate (nitrocellulose or pyroxylin), ethyl cellulose, hydroxy ethyl cellulose, methyl cellulose and sodium carboxy methyl cellulose, ceramic or refractory inorganic adhesives
cyanoacrylates, epoxy adhesives, epoxy nylon, epoxy polyamide, epoxy polysulphide, epoxy polyurethane, fish glue, furanes etc.
The present book covers the manufacturing processes of different industrial adhesives with their formulae. It is hoped that the book can serve to new entrepreneurs, technocrats and existing units to the technology of adhesive and guide them to a useful understanding of the wide variety of adhesives which exist today.
Adhesive Properties and General Characteristics
An adhesive or formulation is
generally a mixture of
several materials. The extent of mixture and the ratio usually depend
upon the
properties desired in the final bonded joint. The basic materials may
be defined
as those substances, which provide the necessary adhesive and binding
properties.
Solvents are employed in many
systems to provide
vehicle and viscosity control. In some cases, low molecular weight
resins of
high fluidity are added to the basic resin to help control viscosity.
Fillers such as metallic
oxides, mineral powders and
various fibers are sometimes used to control reinforcement, decrease
shrinkage,
lower the coefficient of thermal expansion, control temperature
operating
ranges, and in some instances provide a more satisfactory system for a
special
environmental condition. Fillers are also used to control viscosity,
especially
if a thixotropic paste is desired. The most common of these are the
ultrafine
mesh silicas such as Cab O Sil. Most fillers also lower the cost of the
system.
They also prevent waste by virtue of improving the handling properties.
They
are often referred to as extenders.
Catalysts and hardeners are
employed to activate the
resin systems, especially where thermosetting resins are concerned, in
order to
speed up hardening and make the adhesive system practical. Acids,
bases, salts,
alcohols, sulfur compounds, and peroxides are a few of the basic
catalyst
materials. The selection must be based upon knowledge of the mechanics
of
polymerization reactions, which account for the curing or hardening of
adhesives. Catalysts are very important in forming the final joint. The
amount
of catalysis is critical. Overcatalying may result in a poor joint, and
the
same holds true for under catalyzing.
There are several classes,
types, and groups of
adhesives. These have been classified as to use, chemical composition,
mode of
application, setting factors, vehicle, etc. The first general
classifications
to be considered are structural and nonstructural adhesives. These
classifications are sometimes difficult to clarify. A structural
adhesive would
normally be defined as one, which can be employed where; joints or load
carrying members associated with primary design are required. This type
of adhesive
will be subjected to large stress loads. The term structural bonded
joints
equate structural with the importance of its mission. In this concept,
a
further definition may be required where primary structural means loss
of the
aircraft or vehicle through joint failure and secondary structural
means severe
damage and impairment of the mission. The criteria in many cases have
been
defined on the basis of bond strength using the arbitrary top value
strength of
1000 psi.
This is considered by many as
a very poor
definition, and since many people have disagreed on these terms, this
discussion is included only to raise the question and allow the
individual
concerned to draw his own conclusion. It must be considered because a
large
portion of companies has specifications placed in these general
categories. The
major problem is that they all differ in context.
Nonstructural adhesives are
not capable of
supporting appreciable loads and are generally required to locate parts
in an
assembly. They will be employed many times where only a temporary bond
is
required. Their failure would not usually result in the loss of a
vehicle.
Adhesives, sealants, and coatings usually fall into this category, but
could be
responsible for the full accomplishment of the mission.
The type of adhesive material is
easier to define and
usually falls into three categories
1.
Thermosetting resins are synthetic
organic substances, which can be converted by chemical reaction into a
permanently hard, practically infusible, and insoluble solid. These
resins are
high molecular weight polymers, which react by polymerization to form
hard
substances, usually rigid and possessing high strength properties.
Thermosetting resins usually have a high modulus of elasticity, do not
support
combustion, and resist the action of most chemicals. When reacted, the
thermosetting system will not be liquefied by heat but will deteriorate
or
decompose under heat ranges beyond its limitations. We might compare
this to
the baking of bread. Once it is catalyzed (baking powder), further
baking will
only burn it.
2. The
thermoplastic resins are often employed in metal and plastic bonding
and
usually adhere well to both. They do not lend themselves to use as good
load
bearing adhesives, especially if they would be subjected to elevated
temperatures. They will soften when heated and harden when cooled. An
example
here would be butter, placed in a molten liquid state by heating and
becoming
solid upon cooling.
The
more common thermoplastic resins include the polyvinyls, acrylics,
polystyrenes, celluloses, and polyamides. They are sometimes used
effectively
with thermosetting resins for specific formulations.
3.
Elastomeric resins are used widely for modification of the
thermosetting
systems. They generally fall into a distinct class e.g., natural and
synthetic
rubber. A true elastomer is usually defined as a material that will
stretch
twice its original length without inherent loss of elastic properties.
When
used as a modifying agent for other resins, they usually induce
flexibility and
increase peel strength of the systems. They are often used alone or in
slightly
modified form for sealants, but lack the strength to be used alone for
structural applications. Examples of this class are the butyls,
nitriles,
polysulfides, and neoprenes.
Epoxies
A
thermosetting system, 100
percent reactive when in a pure state, the epoxies are very desirable
and more
widely used than any other chemical type. Epoxy is one of the newer
types and
has penetrated more fields of manufacturing operations in a shorter
space of
time than any of its predecessors. The epoxies have been formulated
from more
materials than any other class. They are very versatile and can be
formulated
to do any job, limited only by heat. Some formulations will withstand
800°F for
short periods. The heat ranges of the epoxies are usually determined by
the
catalysts utilized to harden the system. The many catalysts used with
epoxies
produce systems of variable properties. The most common are the
aromatic amines
and cyclic anhydrides. The amines produce the low temperature cure
cycles and
limited heat range, while the anhydrides usually require higher cure
cycles and
withstand higher operational temperatures. Table 1 shows the general
properties
of a basic epoxy resin hardened by various catalysts with 301 stainless
steel
adherends.
Epoxies are available in
liquid, paste, and film
forms (supported and unsupported). The two component systems are more
widely
used because of the extended shelf life. They may be stored for long
periods
and, naturally will not activate until mixed. A few of the early
epoxies were
one part, in a stick form that was heated prior to application, which
proved to
be impractical from an application standpoint. Epoxy adhesives are not
widely
used in the film form unless they are modified, that is, alloyed with
another
adhesive system.
The epoxies are not affected
by bond line thickness
as compared to many other structural adhesives. This is important for
application and processing, because the epoxies require very little
pressure,
and become very fluid when heated prior to the gel or B stage. The thin
bond
line is preferred, but is sometimes difficult to control with a paste
or
liquid. If bond line control is essential, it may be accomplished by
utilizing
glass beads of the desired size in the resin, which do not adversely
affect the
mechanical strength unless used excessively. The bond line control is
one of
the prime advantages of the film type adhesives, especially if a
carrier is
utilized. When utilizing the epoxy with a carrier, care must be
exercised in
pressure application, because bond line starvation will occur, due to
the very
fluid state of the resin under heat and pressure.
The epoxies have low peel and
impact strength as
compared to many other structural adhesives because of their brittle
nature
after cure. To improve the undesirable properties, they are alloyed
with
various other adhesive systems to produce a system to meet the demand
of design
requirements.
Adhesion. The expoxies have high
specific adhesion to
metals, glass, plastics, ceramics, paper, concrete, wood, and various
other
substrates. Because of their brittle nature, epoxies are not
recommended for
bonding the rubbers and elastomeric adherends, although they will
adhere to
these types of materials. The epoxies can be formulated to create
mixtures of
low viscosity and improved wetting, spreading, and penetrating action.
If the
substrate to be joined is cleaned and processed properly, adhesion
presents
very few problems.
Cohesion. When properly cured, the
cohesive properties are
considered very good, but are usually the limiting strength factor. The
adhesive properties are superior to the cohesive properties in most
formulations,
thus cohesive failures will be experienced during testing from room
temperature
to the maximum operating limits of the system.
100 percent solids. The epoxies in
the unmodified state
cure without releasing water or other condensation byproducts. This
makes them
desirable where contact pressures are necessary for manufacturing. They
are
also convenient for bonding such materials as glass or thermoplastics,
where
high heat and pressures would be unsatisfactory. This characteristic
also makes
them desirable as potting compounds, since the possibility of air
bubbles or
inclusions is reduced. The addition of silver, carbon, or other
conductors has
proven very successful in varying the electrical properties of epoxies
without
the problems of discontinuities in the bond line and also without
adversely
affecting the mechanical properties of the system.
Low shrinkage. The epoxies cure
with only a fraction of
the shrinkage of vinyl type adhesives such as polyesters and acrylics
consequently less strain is built into the glue line, and the bond is
stronger.
The shrinkage can be reduced to a fraction of 1 percent by
incorporation of
silica, aluminum oxide, or other organic fillers. A shrinkage factor of
3
percent would be considered extremely high for epoxies.
Low creep. They maintain their
shape under prolonged
stress better than thermoplastics and many thermosetting systems. This
is an
important asset in favor of the use of epoxies, because creep is
considered a
major problem in structural adhesive bonding, and an area of prime
concern by
designers. Creep, in all probability, has hampered the use of adhesives
and
plastics in the building industry more than any other single factor.
Resistance to moisture and
solvents. The epoxies are
resistant to moisture. Moisture does not effect an epoxy in the least
but will
migrate through the joint and deteriorate the substrate. When epoxy
bonded
joints are subjected to moisture or water immersion, the failures
usually occur
at the interface. This indicates the importance of proper surface
preparation
of the adherends. Their resistance to solvents is considered
outstanding and
accounts for their rapid advancement in the coating field. Because
fluids do
migrate through an epoxy with little or no effect to the system the
substrate problem
does exist, Which makes other systems more desirable for use in long
term
exposure to such fluids as fuels, although when modified with an
elastomeric
system, for example, they may possess very desirable properties in
these areas.
Versatility applicable to
modification. The properties of
the epoxy may be changed by
Varying of
the base resin and curing agents.
Varying cure
cycles, both temperature and cure time.
Alloying
the compound with another resin.
Compounding
the various fillers. This may affect the cost factor, but the economics
of
epoxies are governed more by the type of catalyst utilized.
They are effective barriers
to heat and electric
current, yet at the same time may be modified easily for conduction of
electricity. They are versatile in applying due to their wide range of
modification, and may be applied manually, semiautomatically, or
automatically.
Phenolic adhesives
The phenolics or phenol
formaldehyde resins are
formed by the condensation reaction of phenol and formaldehyde. This
material
was discovered in 1872. The phenolics are very rigid, strong, and have
excellent resistance to fungi. They have moderate to good resistance to
moisture, and very good high temperature properties. The phenolic
resins have
been used extensively in the lamination of plywood and in filament
wound
structures. They enjoy a wider range in the structural adhesive
category when
alloyed with other materials.
There are two basic classes
of phenolic resins
resoles and novalacs, and both begin as phenol alcohols. They are
catalyzed
with either an acid or an alkali. Regardless of the formulation of
phenolic
resins, they are considered to have high resistance to deteriorating
influences
encountered in service. They would not be considered excellent in
resistance to
stresses caused by thermal expansion, and extenders should not be used
in
attempts to correct this weakness. When combined or alloyed with other
adhesive
systems, they become excellent structural adhesives and are widely used
in this
manner throughout the aerospace industry.
Nitrile adhesives
The nitrile rubbers are
elastomers and copolymers of
unsaturted nitriles and dienes. The nitriles are not used as structural
adhesives in this form, but yield many one part adhesives that are used
for
bonding small nonstructural parts, especially in the electronics and
plastics
The nitrile rubbers, when prepared for use as a cement, are milled on
tight
cold mill rolls, broken down, and rendered soluble in some type of
solvent. The
most widely used nitrile rubber adhesives are cured by the solvent
escape
drying method, but they may be catalyzed by the utilization of sulfur
compounds
and cured at room or elevated temperatures. The nitriles are available
from the
manufacturers in a variety of formulations, but the important role of
this
rubber system for structural adhesives comes as a result of being
alloyed or
mixed with another resin. The nitriles, like the phenolic resins, do
not have
the desired properties for structural bonding when used alone, but, for
example, if the nitriles and phenolics are combined their mechanical
properties
change to a system with excellent properties for structural use. The
nitriles
give the rigid resins flexibility that produces high peel strengths and
better
than average shear strengths.
Vinyl adhesives
The vinyl polymers do not
stand alone as a
structural adhesive, but hundreds of adhesives are formulated by the
use of
this class of polymer. Vinyl is the univalent radical CH2CH, derived
from
ethylene, a compound which undergoes polymerization to form high
molecular
weight resins. More generally, the term vinyl polymer has been used to
include
a variety of resins, plastic films, and elastomers obtained by
polymerizing
monomers having one or more unsaturated double or triple bonds,
including diolefins,
such as butadiene, vinyldienes such as vinyldiene chloride or methyl
methacaylate, and unsaturated compounds such as maleic anhydride.
The vinyls are important to
adhesive bonding not
only from the adhesive standpoint, but because the films derived from
these
substances are widely used as vacuum bags, slip sheets, etc. The more
widely
used ones are polyvinyl chloride, polyvinyl alcohol, and polyvinyl
fluoride.
Neoprene
Neoprene was the first
synthetic elastomer developed
that possessed properties comparable to natural rubber. It is defined
as an oil
resistant synthetic rubber obtained by polymerizing chloroprene. The
neoprenes
were limited in use due to the cost factor until the shortage of
natural rubber
in World War II. At that time, neoprene was the only synthetic rubber
available
for use in adhesives and as a result, formulators began to experiment
with it.
They found that neoprene adhesives were just as good, if not better in
many
cases, than those based on natural rubber. The neoprenes are used in
three
general capacities in the adhesive industry. They are used structurally
when
alloyed with another resin, as a rubber cement and as a noncuring
tacking
paste. The neoprene cements are usually dispersed in solvents such as
toluene,
which is one of the more widely used. It may be dissolved in mixtures
of
aromatic and aliphatic hydrocarbons.
The neoprene cements may be
cured at room
temperature or by the use of heat, depending upon the accelerator used.
Magnesium and zinc oxides are two of the more common accelerators,
which effect
a slow, room temperature cure.
The maximum operating temperature
does not usually exceed
170°F and would show signs of degradation if used at that temperature
for long
periods of time. As neoprene ages, traces of hydrochloric acid are
formed by
decomposition of the chlorine containing molecules this acid tends to
deteriorate most fabrics such as cotton, rayon, linen, etc.
Polyurethanes
A wide variety of
polyurethanes can be formed by
cross linking highly reactive isocyanates with various polyols. This
elastomeric material provides a bond which resists not only the shear
and
tensile stresses satisfactorily but has very high impact resistance and
excellent cryogenic properties. This has brought them into widespread
use in
space applications, especially for insulation problems. They are also
widely
used as sprayable coatings for aircraft wing assemblies, and for
bonding solid
propellants. They are utilized for bonding metal to metal, elastom
eters, foam,
plastics, nylon, glass, ceramic, and the fluorocarbons. Due to the flow
characteristics they are not considered a good material for honeycomb
construction.
The cohesive strength is
usually better than the
adhesive strength, but good cohesive failures are obtained by careful
processing with a majority of formulations. They yield from 3000 to
5000 psi in
shear at room temperature, but shear strength varies with cure
conditions and
pressure. There is a correlation between the bond line thickness and
shear
strength, the ideal bond line thickness being in the range of 2 to 6
mils. They
will yield up to 8000 psi in shear at 423°F, but are limited to
approximately
250°F at elevated temperatures.
The polyurethanes are not
considered ideal. They
pose processing problems due to their reaction with water and their
gaseous
nature. The systems that are MOCA catalyzed require hot mixing to
diffuse the
catalyst into the resin. The ratio of MOCA to resin has varying effects
on the
final joint and should be carefully controlled. They may be degassed
before
application, but the amount of degassing affects the pot life. When
applied
before gelling starts, they are very fluid and sometimes tend to cause
starved
bond lines. This has been controlled in special applications by the
addition of
6 to 12 percent of nylon fibers.
Another problem associated
with the polyurethane
system applies to storage. Storage must be maintained that will inhibit
fractional crystallization in fact, it should inhibit any
crystallization and
water accrual in the raw material.
In summary, the polyurethanes
are excellent
cryogenic materials, exhibit excellent shock properties, are more
difficult to
process than many systems, suffer from excessive creep at room
temperature, and
show changes in properties on aging, some of which are undesirable.
They still
hold the answer to cryogenic application, but have poor elevated
temperature
strength.
Silicones
Silicones are semi inorganic
polymers made up of a
skeleton structure of alternate silicone and oxygen atoms with various
organic
groups attached, and are thermosetting type resins. A large variety of
the RTV
(room temperature vulcanizing) compounds are formulated utilizing the
silicone
resins. They do not possess the mechanical properties to be used as
structural
adhesives, but are widely used as sealants and potting compounds. The
unit was
designed to pass heat from the electronic equipment to the outside
radiation
system.
The silicones vary in curing
temperatures from room
temperature to 250°F, depending on the formulation and vulcanizing
agent. The
majority of these systems require only contact pressures during cure.
The silicones have many very
desirable
characteristics as listed
1.
Good high temperature
properties. They have good thermal and oxidative stability at
temperatures up
to 600°F and will withstand short exposures up to 800°F.
2.
Silicones are good thermal insulators, which accounts for
their utilization as thermal insulation and heat sinks.
3.
They have good low temperature properties when compared to
many other systems. The methyl silicones have brittlepoints at 100°F.
but the
methyl phenyl silicones may be used to
175°F.
4.
They maintain good electrical properties over a wide
temperature range.
5.
They have adequate resistance to aging and weathering and
remain, stable when exposed to ozone, corona, and sunlight.
6.
They have fair resistance to water and moisture.
7.
Generally,
all silicones will withstand radiation however; the most effective
group is the
silicone resins, followed closely by the silicons rubbers. In all
probability,
the most outstanding characteristic is their ability to resist combined
heat
and radiation.
8.
The ease of handling and low temperature cures brands the
silicones for future growth.
The
silicones
are handicapped by low shear strength and many do not possess the
adhesion or
tack quality level desired. Adhesion may promoted by the use of
primers. They
deteriorate under constant contact with fuels, which limits their usage
in fuel
areas.
Polyesters
The reaction of organic acids
and alcohols produces
a class of materials called esters. When the acids are polybasic and
the
alcohols are polyhydric, they can react to form very complex esters.
They are
usually called alkyds and have long been useful as surface coatings and
glass
reinforced plastics. This same principle, utilized with various
modifications,
brings the polyesters into the adhesive field. The polyester adhesive
systems
cure rigid, and have a temperature operating range up to 500°F. They
reveal
poor adhesion to metals, especially aluminum.
The polyesters are attacked
by most solvents and
have a high shrinkage rate when compared to other adhesives. Shrinkage
rates
may run as high as 4 percent. Attempts are made to combat the high
shrink rate
by the utilization of fillers such as calcium carbonate and aluminum
silicate.
Recently a new polyester has
been developed that is
flexible. The evaluation of this system is incomplete but indications
are that
it adheres better to metals than many of the earlier polyester systems.
Acrylics
The acrylics are a group of
thermoplastic resins
formed by polymerizing the esters or amides of acrylic acid. They are
usually
transparent, low viscosity, polymerizable liquids and were developed
primarily
for use as liquid locknuts. They are now used as adhesives, but are
more important
pertinent to structures as transparent sheets (plexiglass and Lucite).
The acrylic adhesives have
indefinite shelf life
when stored at ambient temperature with access to oxygen. When oxygen
is
excluded by applying the material in a thin film between two mating
surfaces,
gelation occurs at room temperature in a matter of minutes. To prevent
gelation
before application, the liquid is packaged in a low density
polyethylene
container permeable to oxygen. Curing may be accelerated by elevating
the temperature
using an oven, heat lamp, or press. The acrylics have been cured
successfully
in a vapor degreaser when small details are being joined. The heat
causes
gelation to occur before the solvent extracts the adhesive from the
joint.
Perchloroethylene, with a boiling point of 250°F results in a more
rapid cure
with less leaching than trichloroethylene with a 190°F boiling point.
Also, the
vapor degreaser removes the thin film of liquid that is kept from
curing by
contact with the air.
If the liquids are applied to
sensitive electromechanical
devices, be sure the uncured surface liquid is removed. Outgassing and
condensation of volatiles in sealed systems may cause problems in
service or
storage.
Certain metals, such as zinc,
cadmium, and gold, do
not promote cure of these materials. For these metals an organometallic
activator is supplied in solution in a chlorinated solvent, which is
applied
directly to the metal and allowed to dry. Zinc and cadmium plated
surfaces can
also be activated by a chromic rinse prior to sealing the surface.
Strength is far too low for
the acrylics to be used
as structural adhesives in lap joints, but resistance to torque shear
is
outstanding for joining cams, sleeves, pulleys, and gears to shafts in
lieu of
conventional fasteners.
Rosin (sometimes called colophony)
Spirit soluble thermoplastic
materials are available
in two forms gum rosin, the more popular form of which is obtained by
distillation of the exudation fom pine trees, and wood rosin, which is
prepared
from pine trees.
Rosin adhesives are used for
metal container
labeling either as hot melts or in solvent solution, often with added
plasticizers. It is also used in the modification of other resins.
Rosin is
used in the powder form for bonding wood components in aircraft, but is
not
usually termed a structural adhesive. It has very good strength, and
good water
and moisture resistance, but poor resistance to fuels and solvents.
Polysulfide rubber adhesives
The polysulfide adhesives are
synthetic polymers
obtained by the reaction of sodium polysulfide with organic dichlorides
such as
dichloro diethyl formal, alone or mixed with ethylene dichloride.
The polysulfides are used as
adhesives where high
strengths are not required, but are used more often as sealants. They
are
sometimes used as binders for solid propellants. This system offers
good
resistance to light, oxygen, oils, and solvents, and impermeability to
various
gases. They adhere well to almost any adherend, but have poor tensile
properties. They exhibit poor properties when subjected to high
humidity
conditions and have an operating temperature range of –67 to +250°F.
The polysulfides are usually
procured in two
component paste or liquid systems, have good shelf life, and require no
special
storage facilities. They may be catalyzed, mixed, and frozen for
several days
to eliminate production handling problems. The polysulfides are
considered a
wise choice (if the service requirements do not exceed their
capabilities)
because of the economy involved.
Ceramic adhesives
A typical formulation of
ceramic adhesives may
contain silica, sodium nitrate, boric acid, and ferric oxide. These
materials
are heated above 2000°F, blended, and then crystallized. A hard frit is
formed,
dried, milled, and then passed through a screen of the desired size to
ensure
uniform grain size. Oxides and water are then added and the results are
ceramic
adhesives. The viscosity may be controlled by the amount of water added
to the
mixture.
In recent years,
investigations have been carried
out to adapt the ceramic adhesives to structural bonding. They possess
high
shear strengths up to 1500°F (1800 psi in shear) and may reach 5000 psi
in
shear at room temperature.
The prime disadvantages are
low peel and flexural
strengths coupled with very high temperature cure cycles. Much
attention has
been diverted from the ceramic systems since the newer polyaromatics
became a
reality. At present, the ceramics are not practical but research is
currently
in progress to improve the unfavorable properties they possess. The
ceramics
must be improved to become a sound structural adhesive but show great
promise
as an encapsulation material for high temperature rocket nozzles and
nose
cones.
Cyanoacrylate adhesives
A special purpose proprietary
cyanoacrylatc adhesive
(Eastman 910) is a one part, clear, watery liquid. It is free of
solvents and
cures at room temperature in contact with many surfaces without the
addition of
a catalyst or hardener. The system sets by an anionic polymerization
mechanism,
which is catalyzed by weak bases such as traces of moisture on most
surfaces in
contact with the atmosphere. Surfaces such as phenolic, polyester,
polyethylene, and polystyrene plastics tend to inhibit the curing rate,
and may
be pretreated with a diluted solution of an activator,
phenylethylethanolamine
(910 surface activator). However, most common metals, glass, wood, and
rubber
surfaces bond very rapidly and cure in a matter of minutes.
Shear strengths up to 4000
psi can be obtained, but
peel and impact strengths are poor. When exposed to elevatrd
temperatures the
mechanical properties are poor and aging as low as 160°F reflects
degradation.
When exposed to temperatures above this, the adhesives turn yellow and
decompose. The prime advantage is the fast cure, and relatively little
effort
its required for well mated joints. It is widely used for bonding small
electrical components and as a tacking adhesive however, it is
expensive.
Problems have been
encountered on production lines,
as the adhesive presents an operator hazard because of its strong and
rapid
adhesion to the skin. Curing may be slow when the humidity is low, but
this
problem may easily be solved by placing an open container of water near
the
parts being bonded, but this material should never be placed in an oven
for cure.
Adhesive Materials and Properties
THE COMPONENTS OF AN ADHESIVE
The components of the
adhesive mixture are usually
determined by the need to satisfy certain fabrication properties of the
adhesive, or properties required in the final joint. The basic
component is the
binding substance which provides the adhesive and cohesive strength in
the bond
it is usually an organic resin but can be a rubber, an inorganic
compound or a
natural product. Other constituents of the adhesive fulfill other
functions.
Diluent
This is employed as a solvent
vehicle for other
adhesive components and also to provide the viscosity control which
makes a
uniformly thin adhesive coaling possible. Occasionally, liquid resins
are added
to control viscosity.
Catalysts and Hardeners
These are curing agents for
adhesive systems.
Hardeners effect curing by chemically combining with the binder
material and
are based on a variety of materials (monomeric, polymeric, or mixed
compounds).
The ratio of hardener to binder determines the physical properties of
the
adhesive and can usually be varied within a small range. Thus,
polyamides
combine with epoxy resins to produce a cured adhesive. Catalysts, which
themselves remain unchanged, are also employed as curing agents for
thermosetting resins to reduce cure time and increase the cross linking
of the
synthetic polymer. Acids, bases, salts, sulphur compounds and peroxides
are
commonly used and, unlike hardeners, only small quantities are required
to
effect curing. The amount of catalyst is critical and poor bond
strengths
result where resins are over or under catalyzed.
Accelerators, Inhibitors and Retarders
These substances control the
curing rate. An
accelerator is a substance that speeds up curing caused by a catalyst
by
combining with the binder (a catalyst may have the same effect but will
not
lose its chemical identity during the process). An inhibitor arrests
the curing
reaction entirely whereas a retarder slows it down and prolongs the
storage
and/or the working life of the adhesive.
Modifiers
There are many chemically
inert ingredients which
are added to adhesive compositions to alter their end use or
fabrication
properties. Modifiers include fillers, extenders, thinners,
plasticizers, stabilizers,
or wetting agents, and each material is used for a special purpose.
Fillers are non adhesive
materials which improve the
working properties, permanence, strength, or other qualities of the
adhesive
bond and those commonly used are wood flour, silica, alumina, titanium
oxide,
metal powders, china clay and earths, slate dust, asbestos and glass
fibres.
Some fillers may act as extenders.
Extenders are substances
which usually have some
adhesive properties and are added as diluents to reduce the
concentration of
other adhesive components and thereby the cost of the adhesive.
Extenders often
have positive value in modifying the physical properties of the glue
line by
providing reinforcement to resins which would otherwise craze. Common
extenders
are flours, soluble lignin and pulverized partly cured synthetic
resins.
Thinners are generally volatile liquids which are added to an adhesive
to
modify the consistency of other properties. Plasticisers are
incorporated in a
formulation to provide the adhesive bond with flexibility or
distensibility,
Plasticisers may reduce the melt viscosity of hot melt adhesives or
lower the
elastic modulus of a solidified adhesive. Stabilizers are added to an
adhesive
to increase its resistance to adverse service conditions such as,
light, heat,
radiation, etc. Wetting agents promote interfacial contact between
adhesive and
adherends by improving the wetting and spreading qualities of the
adhesive.
ADHESIVES TYPES
One objective of this
handbook is to indicate the
basic properties of the different adhesives types. To a large extent
the
mechanical properties depend on the thermosetting or thermoplastic
nature of
the bond and the following general discussion of these differences
provides
background information to the detailed listing of the adhesives types
(based on
the major chemical ingredient) which follows.
Thermoplastic Adhesives
The thermoplastic adhesives
are classified under the
general categories of thermoplastic resin and thermoplastic rubber
adhesives.
As a class, thermoplastic adhesives are fusible, soluble, soften when
heated
and are subject to creep under stress. Unlike the thermosetting resins,
they do
not change chemically in establishing a bond. The thermoplastic nature
of these
materials confines their application as adhesives to low load
assemblies formed
from metals, ceramics, glass, plastics and porous materials based on
paper,
wood, leather and fabrics and which are not subject to severe service
conditions. Hot melt adhesives, which fall into this class, are being
increasingly employed for fast assembly of packaging materials and
plastic film
laminates.
Thermoplastic resin adhesives
are based on various
synthetic materials (typified by the polyamide, vinyl and acrylic
polymers and
cellulose derivatives) or on natural products such as rosin, shellac,
oleoresins and the mineral waxes. The important hot melt adhesives are
invariably compounded from polyethylene, vinyl polymers and co
polymers,
polystyrene, polycarbonates, polyamides and other polymers. Additives,
including plasticisers, fillers, and reinforcing materials, are
frequently
compounded with the resins to confer particular properties on the
adhesive.
With the exception of pastes, these adhesives are available in the same
forms
as the thermosetting adhesives, i.e. liquid forms can be solutions,
dispersions, or emulsions of the polymer and other modifying components
in a
volatile medium. Solid forms are also available as films (supported and
unsupported), pellets, sticks, or extruded cord lengths, suitable for
machine
application. Other solvent free liquid forms (100% solids systems)
contain the
thermoplastic material as a monomer or as a pre polymer which requires
a
catalyst to bring about polymerization to a high molecular weight solid.
Thermoplastic rubber
adhesives are some of the most
versatile industrial adhesives currently used. The rubber based
adhesives
discussed in the following pages include natural and reclaim rubbers
and
synthetic elastomers such as polychloroprene (neo prene), butyl,
styrene
butadiene and acrylonitrile butadiene (nitrile). Most of the elastomers
are
available in solvent and latex forms or as water dispersions and other
types
are supplied with vulcanising agents. The thermoplastic rubber
adhesives are
generally modified with fillers, plasticisers and compounding
ingredients. The
types of rubber and solvent vehicle used partly determines the physical
and
chemical properties of the adhesives and the many compounding
techniques
employed result in widespread variations in strengths, tack ranges,
drying
rates, environmental resistance and other properties.
Heat or solvent activation is
used to convert film
adhesives into the fluid state prior to bonding. Solvent activation is
applicable only to situations where an adherend is porous enough to
permit
solvent release by absorption and diffusion and heat activation is
employed
where adherends are impermeable and able to withstand the temperatures
involved. Heating also has the effect of curing any thermosetting
component
which may be present in the adhesive. Both techniques are also used
prior to bonding
to activate substrates which have previously been coated with a solvent
base
adhesive and dried to a tack free state. Bonding is usually carried out
under
heat and pressure after joint assembly. Solid thermoplastic adhesives
of the
hot melt type rely on heat to render them fluid and on cooling to bring
about
the setting action.
An unusual cure mechanism is
displayed by the
cyanoacrylates which are an example of chemically blocked materials.
When
confined between close fitting parts, these one component liquid
monomers
undergo polymerization in a very short period (often 15 s). The thin
moisture
film which is usually present on exposed surfaces is sufficient to
harden these
materials if the glue line is thin enough.
Thermosetting Adhesives
As a group, the thermosetting
adhesives form bonds
which are essentially infusible and insoluble through the action of
heat,
catalysts or combinations of these. In contrast to thermoplastics, the
thermosetting resins display good creep resistance and provide the
basis for
many structural adhesives intended for high load applications and
exposure to
severe environmental conditions such as heat, cold, radiation, humidity
and
chemical atmospheres. Thermosetting adhesives include materials of
natural
origin such as animal glues, soybean and vegetable proteins, casein and
miscellaneous water based adhesives as well as synthetic products based
on
epoxy, phenolic, polyester, polyaromatic and other thermosetting
polymers.
Water based adhesives
prepared from low strength
materials of animal or vegetable origin were the earliest adhesives
used and
are still, important for furniture and plywood manufacture, paper and
packaging
materials, and similar applications where low strength and a limited
durability
to outdoor conditions are acceptable. In addition there are
thermosetting
rubber resin adhesives and other blends referred to as thermosetting
thermoplastic resin adhesives. These adhesives have increased toughness
and
strength while their improved resilience enhances stress distribution
properties. Examples of the latter class are the phenolic resins
modified with
nylon or various vinyl resins. The characteristics of these materials
are, in
general similar to those of thermosetting adhesives and hence many of
these
products are employed as structural bonding agents for metal to metal.
Epoxy
resins are modified with poly sulphides to improve their flexibility
and are
thermosetting materials. However, polysulphide adhesives often function
as
sealing materials and may be thermoplastic or thermosetting according
to
formulation and cure.
Thermosetting adhesives are
supplied as liquids,
pastes and solids. Liquid types are generally one or two component
systems
which are already non solvent, containing 100% solids materials, or
react to
become so by catalytic action. Some liquid adhesives contain a volatile
solvent
which is non reactive and which acts as a dispersant or improves the
handling
and processing properties of the system. The curing agent for a liquid
system
may be a powder which requires to be melted before mixing the
components. As a
result of added modifying agents, pastes are usually thixotropic and
may be
applied to vertical joints as non sag adhesives which will not flow out
during
assembly and cure of a bonded structure. Film forms may be supported or
unsupported and of various thicknesses. They have the advantages of
easy, clean
handling and can be cut to conform to the shape of the joint. The shelf
life of
film and one component types is increased by refrigeration but in the
case of
some film adhesives cold storage is essential to prevent room
temperature
curing.
Natural product thermosets,
like animal glue, set by
loss of solvent. Many two component liquid types, such as epoxy resins,
cure by
catalytic action with or without the aid of heat. Other two parts
thermosetting
rubber resin adhesives can be vulcanized at room temperature but
otherwise
curing with heat and pressure is necessary. Some rubber resin adhesives
which
are used to bond unvulcanised rubber to metal cure during subsequent
vulcanization
of the rubber while other adhesives, employed to bond already
vulcanized rubber
to metal, are cured separately. Structural film adhesives invariably
require
heat and pressure to realise the maximum mechanical properties and
curing
temperatures ranging from 150 250°C with bonding pressures up to 100
N/cm2 are
not uncommon. Post cures are often an additional processing requirement
for
structural adhesives where optimum strength is sought.
Rubber Resin Blends
There are innumerable
adhesives in which rubbers and
resins, both natural and synthetic, are blended to obtain combinations
of
desired properties of both types of material. Blended adhesives may be
employed
for structural or general purpose bonding according to the type of
resin and
rubber used and their ratio in a formulation. Those consisting mainly
of
thermosetting resins modified with synthetic rubber are used for the
structural
bonding of metal and other rigid materials. Phenolic nitrile and
phenolic
neoprene adhesives are examples of this type, in which the rubber
component
serves to improve the flexibility of the cured bond and promote its
resistance
to impact or shock loading. Thermosetting resins alone lend to be
brittle.
Adhesives based on rubber, with a certain amount of natural or
synthetic resin
as a modifying component, represent the other end of the scale. In
practice,
the various types of rubber are rarely used alone as adhesives but are
invariably modified with resins to improve such properties, as tack,
cohesive strength,
specific adhesion to surfaces and heat resistance. Within these
extremes are
numerous formulations in which various ratios of resin to rubber are
used.
These adhesives have a wide range of applications which include bonding
of
textiles bonding of synthetic fabrics to wood and metal affixing
wallboards and
tiles lamination of paper, metal foil and plastic films laying of
flooring
materials and various other industrial or domestic applications.
The structural rubber resin
adhesives are available
as films or tapes (supported or unsupported on fabric carrier cloths)
and
occasionally as solvent solutions. The films are cured at elevated
temperatures
up to 200°C and under bonding pressures ranging from 30 100 N/cm2. Post
curing
is often included to ensure optimum mechanical properties for the cured
adhesive. Liquid types are dried to remove solvent and then processed
as film
ad hesives. The non structural rubber resin adhesives are generally
supplied as
solutions in organic solvent mixtures and can be applied by brush,
spray, dio
or roller coater, spatula, or flow techniques. Because these adhesives
rely on
a loss of solvent before adhesive action can take place the shelf life
and
working life are usually indefinitely long provided the solvent content
is maintained.
With porous adherends the assembly can be made with wet adhesive and
time
allowed for solvents to escape by diffusion through the material. Where
solvents have a high volatility assembly times may be as short as 15
min by
which time the substrates have lost the tackiness required for contact
bonding.
Impermeable materials are coated with adhesive and bonded together only
after
the bulk of the solvent has been dried off to leave the adhesive in a
tacky
state. Light assemblies can frequently be handled after a few hours but
heavier
assemblies require a setting period of at least 24 h. Maximum joint
strength is
not realised until after a few days following the removal of residual
solvent
traces. Wet bonding generally produces joints having good strength and
durability but poor solvent resistance. Optimum performance is given by
heat
curing (according to manufacturers instructions) which has the effect
of
removing the trace solvents otherwise retained by these adhesives and
which act
as plasticiscrs and increase the thermoplasticity of the system. Heat
also
promotes cross linking of the adhesive constituents and thereby
increases the
creep resistance of the joint. Processing conditions depend on the
adhesive
with bonding pressures ranging from 10 300 N/cm2 according to joint
factors
such as rigidity, dimensions, closeness of fit and glue line thickness.
Low
bonding pressures are more satisfactory for glue lines exceeding 0.2
mm. Curing
schedules range from 1 h at 80°C to 20 30 min at 140°C, with optimum
properties
resulting from the longer curing periods.
PROPERTIES OF BASIC ADHESIVES TYPES
This section has been
prepared almost entirely from
published material appearing in technical books and journals. The
length of an
entry is not indicative of the importance of the adhesive type under
consideration since the amount of information available was found to
vary
considerably. Due regard has been paid to technical information in the
trade
literature received from the various adhesives manufacturers.
Manufacturers
literature was found particularly useful for confirmation of such
adhesive
properties as colour, available form, processing factors, and
applications.
These data necessary to supplement material from published sources and
effort
has been made to keep the section free from any trade bias.
It has already been noted
earlier that adhesives
based on the same material may show considerable variation in their
properties
where modifying materials have been added to the formulation.
Properties are
dependent, not only on the adhesive composition, but also on the
conditions
under which it is prepared and used. Because of these possible
variations any
values given in this section should be regarded as representative of
the
probable behaviour of a basic type of material used under certain
conditions.
This is further complicated by the fact that a large number of
commercial
adhesives are blends of two or more basic adhesive types in particular
both
natural and synthetic rubbers and resins are often used together and
form the
basis of the numerous resin rubber or rubber resin adhesives which are
available. Consultation with the manufacturers concerned is strongly
recommended where detailed information is required on the behaviour of
specific
commercial adhesives under various service conditions.
Acrylics
Type
Thermoplastic resins based on
acrylates (properties
of polymethyl methacrylate types are discussed below) or derivatives
(amides
and esters).
Physical form
Available as emulsions,
solvent solutions, and
monomer polymer mixtures (one or two components) with catalysts (liquid
or
powder). One component liquids which polymerize under ultra violet
radiation
are available.
Setting action
Emulsion solvent types set by
evaporation and
absorption of solvent. Polymer mixtures set through polymerization by
heat,
ultra violet radiation and/ or chemical catalysts.
Processing conditions
Solvent types set over a
period of 20 days at 20°C
or 6 h at 80°C. Polymer mixture setting times depend on polymerizing
method
used chemical action, 14 d at 20°C or 4h at 80°C ultra violet action, 5
h
exposure heal action, 2 h at 5.5° C followed by 8 h at 80°C. Bonding
pressures
range from contact to 17 N/cm2.
Permanence
Resistance to weathering and
moisture varies from
poor (solvent types) to excellent (polymer mixtures). Change from
transparent
to yellow colour may occur with time (over 1 yr period). Not affected
by
alkalies, non oxidizing acids, salt spray, petroleum fuels but attacked
by
alcohols, strong solvents and hydrocarbons (aromatic and chlorinated).
Highly
resistant to ultra violet exposure. Service temperature range of
acrylic resin
adhesives is 60°C
to 52°C.
Applications
Light structural assemblies
based on acrylic
plastics to themselves, wood, glass, metals, rubber, leather and
fabrics colourless
jointing of decorative plastic laminates production line assembly of
components
(ultra violet effective here) outdoor applications such as plastic name
plates
aluminium foil work windshields, instrument panels, lenses and optical
components in aircraft, marine, and automotive industries. One type, n
butyl
methacrylate (alone or modified with Canada Balsam) is used as an
optical
cement. Ross 24 (modified type) is a heat setting cement which is
transparent (=
1.485) and has good thermal shock resistance.
Physical Testing of Adhesives
INTRODUCTION
A detailed description of the
various test methods
that have been developed for adhesive bonds is beyond the scope of this
handbook. A short outline, in chart form, of commonly used lest
specimens
follow the remarks on the evaluation of adhesive strength. Non
destructive test
methods and the effects of adverse service conditions on bond strength
are
dealt with in subsequent paragraphs and the section concludes with a
list, of
titles of widely accepted standard test methods.
STRENGTH PROPERTIES
Specialised testing methods
are required for the
evaluation of the strength properties of adhesives. In addition to
joint
strength determination these methods provide a means for checking the
efficacy
of the processes used to make the bonds. The joint strength is
invariably
dependent on bonding technique factors such as adhesive application,
adherend
pretreatment and the adhesive curing conditions. Bonding conditions
also
determine the repro ducibility of test results and complete information
on a
number of variables is, therefore, necessary before undertaking an
adhesive
evaluation. The following particulars are essential for the fabrication
of
reliable test specimens.
Instructions
for the preparation of the adhesive
Adherend
surface pretreatment procedures recommended for the adhesive under
consideration. (Special treatments may be involved where certain
environmental
tests are envisaged)
Adhesive
application and processing before bonding. (Attention to coating
thicknesses
and their control, or drying conditions is often important).
Manufacturers
specified conditions for joint assembly (temperatures, humidities or
times)
Adhesive
curing
conditions relating to bonding temperatures, pressures and times.
Test specimens required to
give reproducible failing
strengths need to be carefully designed and prepared unsatisfactory
bonds will
result from the faulty execution of any stage in the assembly process.
Most of
the standard test methods employ test specimens of definite shape and
size,
which have to be machined to specified tolerances. Most methods specify
the
number of specimens to be tested in order to obtain a reliable result
because
testing factors and slight differences between adhesive batches
prepared under
identical conditions lead to joint strength variations. Ten or more
specimens
may be required to give meaningful data. The equipment used for testing
is
important and will influence the reliability of strength values
obtained. Some
variations in the performance between machines of the same type are to
be
expected. Often, test machine accuracy is greatest over a limited
working range
of the loading capacity (usually 10 90%), and test specimens should
fail at
loads within this span. The rate at which test specimens are stressed
is
another factor influencing the strength values obtained for adhesive
bonds.
Standard test methods generally specify the testing rate although it
may be
better to adopt a rate, which more closely resembles the stressing rate
that an
actual assembly is likely to experience.
The data obtained from test
methods is useful for
comparing the performance of several adhesives prior to the selection
of one
for a particular assembly job. It must be emphasised that the test
specimen
rarely simulates the actual configuration of an assembly and that the
test data
cannot therefore, be relied upon to predict the performance of the
assembly in
service. The same limitation applies to test specimens removed from an
assembly
these are unlikely to represent the behaviour of the whole structure.
Short of
testing an assembly under service conditions it is necessary to adopt a
test
specimen and method, which simulate the assembly and its working
environment as
closely as is practicable. The testing procedure finally employed must
produce
results that are likely to show a good correlation with the results
that would
he obtain in tests on assemblies. In this respect, selected standard
test
procedures are frequently employable without modification.
ASSESSMENT OF DURABILITY AND STRENGTH PARAMETERS
Fatigue
Fatigue testing refers to the
repeated application
of a specified load or deformation on a bonded specimen. Tests may be
conducted
under static or dynamic conditions (or both separately if necessary)
according
to the data required to evaluate an adhesive under service conditions.
Static fatigue properties are
determined by
measuring the maximum loading sustained by an adhesive over a given
time.
Various weight loadings applied to shear or tensile specimens provide a
measure
of the time required for bond failure.
Dynamic fatigue properties
are measured by cycling
test specimens with specified minimum to maximum stress loading for a
given
period or number of cycles or until failure. Cycle frequencies usually
vary within
the range 5000 to 107 Hertz. In addition to frequency, fatigue life is
determined by amplitude, temperature and mode of stressing these
variables must
be specified along with the extent of loading. These tests do not
determine the
damping properties or elastic moduli of adhesives.
Creep
There is no standard test for
measuring the
distortion or dimensional change in a bonded specimen under sustained
loading
(Creep). Deformation of the adhesive is generally measured by noting
the
dimensional change occurring when a bonded specimen is subjected to a
constant
load for a specified time and temperature. Room temperature creep is
known as
cold flow. Higher temperatures usually increase the rate of creep
significantly.
Creep tests are often carried
out to determine joint
deformation when stressed below the failing load required to break the
bond.
Joints may be loaded by springs (ASTM D2294 64T) or dead weights (MIL A
5090E)
to maintain constant loading in a specified environment. Optical
measurement of
the shift in scribed reference lines on a lap joint edge is a useful
method of
creep assessment. Alternatively, the relaxation, or the ability of an
adhesive
to restore to its former state, may be optically determined on removal
of
stress. Rigid thermosetting adhesives display little or no creep under
stress
in contrast to thermoplastic or plasticised adhesives. Prolonged
stressing of
thermoplastic adhesives always reduces bond strength.
Flexural Strength
The shear strength of beams
composed of adhesive
laminated strips may be determined by flexural loading. The load is
applied to
the mid span to develop maximum shear stress and delamination in the
centre
layer of adhesive. The method gives higher shear strengths than are
obtained
with tensile or compressive shear specimens because the resistance of
the
adhesive to shear failure is increased by the compressive loading
normal to the
glue line.
Peel Strength
Peel tests involve complex
stress distributions.
Peel strengths vary with the speed of testing (particularly with low
modulus
adhesives) and the forces needed to start and sustain peeling action
are
determined by the physical properties of the adherends, test specimen
geometry
(adherend thickness and width), and the adhesive strength
characteristics. Peel
strength increases with adherend thickness and adhesive thickness but
decreases
with adhesive modulus of elasticity. Steel adherends give higher peel
strengths
than aluminium adherends of similar thickness. Low peel strengths are
usually a
feature of brittle adhesives with high tensile strengths. Peeling rates
of 15.2
cm/min for adherend widths of 2.54 cm are commonly specified.
Durability
The test specimens described
previously may be used
to determine the effects of adverse environments on an adhesive bond
although no
single test (or series) exists which will enable the user to predict
its
service life. A suitable test will provide information on the
permanence of the
bond when it is exposed to deteriorating circumstances such as
temperature
changes leading to oxidation, thermal degradation or softening of the
adhesive.
Other destructive hazards include low temperatures, sunlight and
radiation,
water, chemical reagents, oils and biodeterioration. Test specimens and
procedures should be selected to simulate the type of service
conditions
envisaged for the bonded assembly. Consideration must be given to the
adherend
material for certain tests, e.g. for the evaluation of the acid
resistance of
an adhesive, certain metals would be unsuitable as adherends. Some of
the environments
which often provide the basis for unfavourable long term conditions of
exposure
for adhesive bonds are discussed here.
Temperature
Adhesives and adherends are
affected by high, low
and varying temperatures. Elevated temperatures may decompose adhesive
materials by oxidation or thermal degradation. Long exposure to
moderate
temperatures often leads to polymerization changes in adhesives.
Displacement
of bonded surfaces occurs where high or low temperatures accentuate
differences
between the thermal coefficients of expansion for adherends and
adhesives
stresses are set up at the interface which influence bond strength. Low
temperatures embrittle many adhesives causing a reduction in their peel
and
cleavage strengths.
Test chambers with heating or
cooling units can be
employed for environment simulation. Adhesive durability is best
determined at
the service temperature higher test temperatures should be regarded for
their
comparative value only. Destructive temperature effects may become
apparent in
hours or years.
Weathering
The
long term ageing or
weathering properties of bonded structures are difficult to predict
since there
are no standard short term permanence tests. Actual long time
weathering is
usually a reliable guide to adhesive durability although variations in
exposure
conditions over long test periods can make data difficult to interpret.
Rainfall, humidity and temperature vary widely with locality.
Accelerated
weathering tests designed to reduce the long exposure periods are
useful if the
results can be correlated with actual weathering. Several tests have
been
adopted in the U.K. for providing a uniform testing procedure for
military
equipment. Referred to as I.S.A.T. (Intensified Standard Automating
Trials),
these tests are claimed to be equivalent to prolonged storage under
existent
weather conditions.
Chemical
The
permanence
of a bond may be affected by exposure to external chemical agents or by
the
latent chemical reactivity of the adhesive for an adherend.
Several tests have been specified
to evaluate adhesive
bond strength on exposure to reagents such as acids, alkalies, water,
sea
water, petrol, organic solvents and lubricating oils. The deterioration
in
adhesion is sometimes dependent on reagent concentration. Temperature
and
exposure period should also be considered as test factors. Other tests
are
concerned with the effects of atmospheric constituents, which are known
to
cause adhesive deterioration, e.g., salt spray or ozone.
Chemical constituents in the
adherends, such as
plasticisers, can migrate into the adhesive and destroy adhesion.
Additionally,
the byproducts of an adhesive curing reaction may attack the adherend
at the
interface and cause loss of adhesion.
Biological
Certain adhesive types based
on natural products
such as casein, cellulose, dextrin or protein, etc., are subject to
attack by
bacteria, fungi, insects and rodents. Tests are available to check the
effectiveness of preservative agents for adhesive formulations
otherwise
subject to biodeterioration.
Radiation
The effects of light, either
artificial or natural,
on bonded glass or optical assemblies, involving transparent or
translucent
materials, may be important. Adverse effects include the loss of
adhesive
strength and the discoloration of the glue lines following
photochemical
changes in the adhesive. Light is unlikely to present a hazard for
impervious
adherend structures but is often an important factor with glass and
transparent
or translucent plastics.
Nuclear radiation is known to
effect structural
changes in high molecular weight polymers but has been scarcely studied
for
adhesive systems. The advent of nuclear technology and space research
can be
expected to produce test methods for this type of environment soon. An
extensive literature on radiation induced changes in polymers provides
a basis
for studying adhesive performance. Some selected references appear at
the end
of the section.
Polyvinyl Acetate Wood Adhesives
Introduction
Polyvinyl acetate is a
thermoplastic polymer that
has gained wide acceptance over the years as a raw material for the
adhesives
industry. Modified or unmodified, in solution or emulsion form, and as
homopolymer or copolymer, it exhibits a versatility that makes it
suitable for
bonding a wide variety of substrates. In particular, it is capable of
producing
strong and durable bonds on wood and wood derived products, and this
has been a
major contributing factor to the tremendous growth of polyvinyl acetate
based
adhesives in recent years, from almost nothing in the early 1930s to an
estimated
worldwide production of all types of 2 million tonnes in 1977. It is
familiar
to people all over the world as the binder for interior and exterior
emulsion
paints, as the so called cold glue that replaced the heated pot of
animal glue
for carpentry, and as the white glue used in millions of households as
a
general purpose adhesive.
Background
Vinyl acetate is a colorless
flammable liquid with a
viscosity of 0.4 cP at 20°C, a solubility in water of about 2% at 25°C,
a
boiling point of 72.7°C, and a characteristic odor. This is the
starting point
for the production of polyvinyl acetate (PVA), and, in conjunction with
other
vinyl monomers, acrylic esters, dialkyl maleates and fumarates,
ethylene, and
certain other monomers, of a range of specialty copolymers and
terpolymers,
while graft polymers can be produced with monomers such as styrene that
will
not copolymerize with vinyl acetate.
It is not certain when the
first polymerization of
vinyl acetate to polyvinyl acetate was performed. During the period
between
1915 and 1925 the free radical initiation of polymerization of various
vinyl
monomers was widely studied and by 1930 polyvinyl acetate was
commercially
available. It was, however, only in the years following World War II
that
polymers of vinyl acetate began to be used in significant quantities,
particularly in the paint and adhesive industries, and since then, PVA
has
shown the same rapid expansion as most other well known thermoplastic
materials. Today some 50 major manufacturers and countless small
suppliers
around the world make material available in solid form, dissolved in
solvent,
or dispersed in water, as homopolymer, copolymer, or terpolymer, for a
variety
of applications that include paint manufacture, production of general
purpose
and specialized adhesives, textile coating, sizing and sealing of paper
and
related products, concrete additives, and production of sealants.
Formulated
products span the entire range of viscosities, from thin liquid to
heavy paste,
and may be fast or slow drying. Dried films may be clear or opaque,
pigmented
or unpigmented, flexible or brittle, hard or soft, as required. The
product may
be internally or externally plasticized or unplasticized, and may dry
hard and
tack free or pressure sensitive. With this versatility it is hardly
surprising
that polyvinyl acetate is now available almost anywhere in the world,
with
factories in most developed countries. At this point, the United States
and
Germany are the major producers, with companies such as Air Products,
Borden,
Monsanto, and Union Carbide in the United States, and Hoechst in
Germany.
Chemistry of Polyvinyl Acetate
The steps involved in the
manufacture of polyvinyl
acetate are shown schematically in Figure 1.
A. Production of Vinyl Acetate Monomer
Originally, vinyl acetate
monomer was produced by
reacting acetylene and acetic acid together with suitable catalysts.
The earlier technique was to
carry out this reaction
in liquid phase but this method was replaced in the early 1930s by more
efficient gaseous phase processes performed at high temperature.
This process remained viable
for as long as calcium
carbide was readily and cheaply available. With the increasing cost of
energy,
however calcium carbide has become steadily more expensive and less
readily
available, while an ever increasing range of chemicals has been made
available
by the rapid expansion of the petrochemical industry. These two factors
have
combined to make the modern method of production increasingly
attractive. Here
ethylene is used as the starting point for the production of both
acetylene and
acetic acid. Acetylene is made by removing hydrogen from ethylene,
while acetic
acid is obtained by oxidizing the ethylene to acetaldehyde, which is
oxidized
further to acetic acid.
B. Polymerization of Vinyl Acetate
Vinyl acetate monomer may be
polymerized by many of
the conventional polymerization techniques, including mass
polymerization, solvent
polymerization, and emulsion polymerization. The reaction is usually
initiated
and controlled by the use of free radical or ionic catalysts, although
experimental methods of catalysis, including redox catalysis or
activation by
light, may be used for specialized products. The polymerization is
characterized by three successive stages of reaction initiation, the
growth of
the polymer, and termination.
1. Initiation
Initiation occurs when a free
radical or ion
attaches itself to a vinyl acetate molecule. This leads to a
rearrangement of
the electrons in the double bond, transferring the reactive site to the
vinyl
acetate monomer.
The initiator is usually a
free radical derived from
a peroxide such as benzoyl, lauroyl, or even hydrogen peroxide,
although other
initiators, such as persulfates, may also be used.
2. Polymerization
This highly reactive
initiated molecule reacts with
further monomer molecules by the same transfer mechanism, retaining the
terminal reactive site for further growth.
3. Termination
Growth of the macromolecule
is terminated when the
reactive site is removed, either by combination with the reactive site
of some
other molecule, or by transfer of the reactive site to some other
molecule.
Selection of the initiating
catalyst, the ratio of
catalyst to monomer, and the reaction conditions allows control over
the
average molecular weight of the polymer formed, and also of the degree
of
branching, if any, in the macromolecule.
In the case of polyvinyl
acetate the polymerization
may be carried out by a wide range of techniques, including mass
polymerization, solution polymerization, and emulsion polymerization.
Most of
the poly vinyl acetate produced is made using emulsion polymerization
techniques, and this is particularly true of those grades used in the
production of wood adhesives. Vinyl acetate is particularly suited to
emulsion
polymerization, owing to the relatively high solubility of the monomer
in
water, and the complete solubility of the polymer in the monomer. In
this
process the vinyl acetate monomer is dispersed by means of relatively
high
speed stirring in water that contains suitable emulsifiers or
protective
colloids. A more or less stable suspension of monomer particles will be
formed
in the water. To this suspension the initiator is added, and typically
the
mixture will be heated to a temperature, which will substantially speed
up the
rate of reaction, while allowing it to remain controllable. As
polyvinyl
acetate is soluble in the monomer, the reaction will take place within
the
individual droplets of the suspension, producing a stable emulsion of
polyvinyl
acetate. Typical commercial polymer emulsions will contain between 40
and 60
parts by weight of the polymer, a very low residual level of monomer,
and a
viscosity between 0.1 and 20 Pa sec. In addition, emulsions intended
for use as
the basis of formulated wood adhesives will also have a relatively
large
particle size, usually within the range 0.3 5 × 10 6 m.
Other characteristics of the
polymer emulsion that
will goven its suitability for use in a specific application will
include the
molecular weight degree of copolymerization or plasticizing, if any
film
strength and film forming properties at low temperature and ability of
the
emulsion to withstand both the mechanical effects of mixing and changes
in
temperature, especially where freezing may be involved. Manufacturers
of PVA
emulsions will supply most or all of this information for their
products to
assist in the selection of the most suitable grade.
Formulating a Pva Based Adhesive
A. General Considerations
Formulating a PVA based wood
adhesive, a number of
factors, sometimes conflicting, must be borne in mind. It follows that
the
final product will often be a compromise in which the conflicting
factors have
been carefully considered in order to give most weight to those, which
seem to
be the most important in the specific application. Factors to consider
will
include the following
1. Substrates
Where the adhesive is to be
used for bonding wood to
wood, consideration must be given to whether the wood will be hardwood
or
softwood. For the hardwoods an adhesive of high solid content is
usually
advantageous, while with softwoods, adhesives of lower solid content
may be
used. In addition, if the species to be glued is known to be oily,
incorporation of a wetting agent or solvent will assist in adhesive
penetration.
In many cases, however, wood
will be only one of the
substrates. The other substrate can vary from concrete in the case of
adhesives
for parquet or mosaic wood blocks to decorative laminates which may be
cellulosic or plastic. Although, in general, PVA adhesives are used
principally
on cellulosic materials because of their exceptionally good adhesion to
such
surfaces, special applications may call for wood to be bonded to
rubber, to
foams, both flexible and rigid, to synthetic or natural fibers, or even
to
metal or other nonporous surfaces. Each of these will impose
restrictions,
sometimes severe, on the freedom of the formulator, and corresponding
limitations on the applications for which the formulated adhesive may
be
suitable.
2. Surface Preparation
The preparation of the
surfaces to be glued will
also influence the formulation to a certain extent. In the case of
adhesives
for gluing wood to wood, inaccurately machined surfaces may necessitate
the
formulation of gap filling adhesives in order to produce a satisfactory
bond
over the entire surface. Adhesives may need to be formulated to give
good
adhesion to greasy, loose, or dusty surfaces or to seal a very porous
substrate.
In certain applications the adhesive must be capable of bonding
surfaces that
have been coated or lacquered,
3. Application
The viscosity of the
formulated adhesive will
largely be governed by the method of application of the adhesive.
Application
methods will include manual application by brush, roller, smooth or
notched
trowel or spray, machine application from smooth or embossed rollers,
with or
without a doctor blade, or by cascade coaters, by nozzle or jet, or
even
mechanical extruders or sprays. Consideration should always be given to
making
the adhesive as easy to use as possible, especially where it will be
handled by
unskilled people unfamiliar with the proper handling of adhesives. This
applies
particularly to adhesives intended for household use.
4. Assembly Conditions
Intricate or multicomponent
assemblies will demand
an adhesive with a long open assembly period in order to enable all the
components to be brought together and placed under pressure before the
adhesive
has started to dry. At the other extreme, applications such as core
composing,
the pressure station is usually very short, require an adhesive that
can
develop high bond strength very quickly. High production rates will
similarly
demand a quick setting adhesive. Application conditions involving high
or low
temperatures or humidities will also influence the formulation. The
formulator
must also consider whether or not the glued article requires machining.
If this
is the case, fillers must be chosen carefully or eliminated in order to
minimize damage to cutters. Adhesives containing solvent should only be
used in
well ventilated locations.
5. Service Conditions
Although PVA adhesives are
not, in general, used for
joints that are under continuous load or subjected to high temperatures
or high
humidity, these adhesives can be formulated to give better performance
under
such conditions. The conditions under which the completed assembly will
be
expected to operate should always be taken into account when designing
the
adhesive.
6. Appearance
Again, the use to which the
completed article is to
be put may influence the formulation. The appearance may be marred by
unsightly
glue lines, in which case it may be necessary to design the adhesive to
dry
completely transparent, or even to tint the adhesive so that the glue
line will
be less obtrusive. Tinting or pigmenting the adhesive may be essential
the
finished article is to be stained, as squeezed out adhesive may seal
the
surface in the vicinity of the glue line and prevent subsequent
penetration of
stain in that area.
7. Storage Conditions
If the adhesive is likely to
be stored under adverse
conditions, attention must be given to ensuring that it has adequate
freeze
thaw stability. The maximum storage life of the adhesive.
8. Price
Very often the most serious
limitation will be that
of formulating to a particular price. In this regard, ease of
application,
reliability, and spread rate are factors to take into account, as it
may be the
case that a relatively expensive adhesive will prove more economical in
the
specific application than an inferior but cheaper product. When
comparing
prices it is important to take the density of the products into
account,
especially if they are sold in units of mass.
B. Formulating and Compounding
A number of different
components will normally be
incorporated into a PVA wood adhesive. Each of these has a specific
function in
the finished product. To formulate successfully, it is necessary to
understand
not only the performance criteria, but also the function of these
components.
1. The Base Polymer
Since the PVA emulsion
provides the major proportion
of the adhesive strength, and will in many cases be the only binder in
the
formulation, it is worth considering its function in some detail. While
the
mechanism of adhesion is not fully understood, adhesion probably occurs
as a
result of secondary forces, principally Debye forces and London
dispersion
forces operating at close range, with some hydrogen bonding with the
cellulosic
fibers especially where polyvinyl alcohol has been used as the
protective
colloid. In addition, mechanical bonding occurs as a result of adhesive
penetration into the open cell structure of the wood. All these combine
to
produce an excellent bond, which in a properly formulated adhesive will
be stronger
than the wood itself.
Since the PVA is in emulsion
form, coalescing of the
particles must occur in order to produce a continuous film during the
drying process;
this will happen only if the drying takes place at a temperature above
the
solidification temperature of the polymer particles. If this condition
is not
met, the particles will not coalesce properly, leading to a loss of
mechanical
properties in the dried film. If the temperature at which evaporation
of the
water takes place is significantly below this minimum film forming
temperature
or white point, no coalescing will take place, and a white, chalky film
will
result with no mechanical strength whatsoever. This white point is thus
an
important aspect of the base PVA to consider when formulating a wood
adhesive.
Viscosity of the emulsion
will be determined by the
solid content of the emulsion, the particle size distribution, and the
emulsifier or protective colloid system used, and may vary between very
wide
limits. For production of wood adhesives it is most common practice to
use
grades with a coarse to medium particle size (in the range 0.3 10 × 10
6 m) and
a solid content between 40 and 60% by weight of the total emulsion.
Even with
these restrictions viscosity may still be anywhere between 0.05 and 50
Pa sec,
however, and it will usually be necessary to modify this viscosity in
the
finished product.
The resistance of the dry
film from a PVA emulsion
to water is mainly dependant on the type and quantity of protective
colloid
used. Where this is polyvinyl aocohol, the water resistance of the
dried film
is generally poor. It is possible, however, either by using emulsions
protected
by cellulose based colloids, or by incorporating certain additives, to
produce
PVA adhesives that have a fair resistance to water.
Because polyvinyl acetate is
a thermoplastic
polymer, it loses cohesive strength as the temperature increases. In
general,
higher molecular weight polymers lose less strength at elevated
temperatures
than those of lower molecular weight, but the differences are not
great. In
addition, thermoplastic polymers are subject to cold creep, which is
the
tendency for a fully dried film to flow slowly under a sustained load.
Plasticized grades are much more subject to cold creep than are
unplasticized
grades. To a certain limited extent, the tendency to cold creep can be
reduced,
but in general PVA wood adhesives are not suitable for applications in
which
the glue line is highly stressed or subject to temperatures above about
50°C
and a combination of these two factors will completely rule out the use
of PVA
wood adhesives.
Different grades of PVA
emulsions will also have
different drying times and will therefore offer the possibility of
formulating
adhesives with long or short open assembly times. All the larger
manufacturers
issue comprehensive data sheets for their various grades from which it
is
possible to select, with a good deal of precision, the best grade to
use as a
starting point.
2. Other Binders
In addition to the PVA
emulsion it is common practice
to incorporate other binders into the formulation of various purposes.
Probably
the most widely used cobinder is polyvinyl alcohol. Incorporation of
polyvinyl
alcohol into a formulation increases the cold creep resistance of the
dried
film, but reduces its water resistance, especially where the polyvinyl
alcohol
has a low degree of hydrolysis. In addition, polyvinyl alcohol will
increase
the open assembly time of the adhesive substantially. Use of high
molecular
weight polyvinyl alcohols is common in low cost formulations, as these
produce
relatively high viscosity solutions allowing incorporation of extra
water into
the formulation. Adhesives containing polyvinyl alcohol will exhibit
good
machine stability and running properties and faster initial bond
strength
development.
The use of starch as an additive is
also common practice.
Here the major advantage is the cost reduction, which is again achieved
at the
expense of the water resistance.
Because of their affinity for
water, starches will
also extend significantly the open assembly time of the adhesive. As
starches
are particularly susceptible to microbial attack, care must be taken to
ensure
that the formulated adhesive is adequately protected. A wide variety of
starch
types may be incorporated; including pre gelatinized, water soluble,
and
oxidized starches. Where borax is used to solubilize the starch, the
compatibility of the starch solution with the base emulsion must be
checked.
Borax has the effect of insolubilizing poly vinyl alcohol, and may
therefore
destroy the protective colloid of the PVA emulsion, thereby
destabilizing the
emulsion.
While cellulose derivatives
such as carboxymethyl
cellulose are often added, their function is usually that of viscosity
modifier
rather than additional binder, and the proportion is invariably small.
Thermosetting resins such as phenol, resorcinol, or urea formaldehyde
resins
are occasionally added to PVA wood adhesives to improve their water
resistance.
Their use will be more fully discussed subsequently. For specialized
application, a range of other binders may be incorporated, especially
where
wood is to be bonded to some other substrate. Thus all or part of the
PVA
homopolymer may be replaced by an ethylene vinyl acetate copolymer for
the
lamination of polyvinyl chloride film to wood, while vinyl acrylate
polymers or
copolymers may be added to improve adhesion to nonporous substrates.
While
dextrins may be added to PVA emulsions used in packaging applications,
they are
not normally added to PVA wood adhesives.
3. Plasticizers
Plasticizers may be regarded
as high boiling
solvents with very low vapor pressures at the operating temperature of
the
adhesive. They will thus remain permanently in the dried film.
Plasticizers
form a film around the particles of the dispersion, increasing the
distance
between them and thus lowering the forces between them. In addition to
increasing the flexibility of the dried film, they also lower the
minimum film
forming temperature of the adhesive. However, they also increase the
tendency
of the film to creep under load and should thus be used with caution in
wood to
wood adhesives. They should be avoided completely in adhesives for
critical
applications, especially highly stressed structures, where the use of a
solvent
to promote film forming characteristics is preferred.
One of the substrates is
flexible, they may be used
to improve adhesion and match the characteristics of the adhesive film
more
closely to those of the substrate, especially where this is flexible.
Only
plasticizers that are compatible with PVA should be used. Those in
common use
include esters, particularly alkyl phthalates such as dibutyl
phthalate, and
aromatic phosphates such as tricresyl phosphate, which are chiefly used
where
flame retardancy is a consideration. Speciality Plasticizers are not
normally
used in wood adhesives. Some physical properties of plasticizers in
common use
are shown in Table 1. The commonly used plasticizers are more or less
immiscible with water; the addition of plasticizer to the base emulsion
does
not present undue difficulty. It is good practice to add the
plasticizer slowly
while stirring the emulsion vigorously. Once added, the stirring rate
may be
decreased, but stirring should be continued for at least 30 min to
ensure
thorough dispersion and to allow the plasticeser to solvate the
emulsion
particles. Plasticiser will seldom be added at a level above 10% based
on
polymer solids in formulations for wood adhesives.
Aminoresin Wood Adhesives
Introduction
Aminoresins are polymeric
products of aldehyde
reaction with compounds carrying – NH2 or –NH groups. Such groups are
mainly
amide groups, such those in urea and melamine. They constitute the most
important members of this class of compounds, more so than the amine
groups as
in the case of aniline. Formaldehyde is the main aldehyde used. Other
aldehydes, such furfural, are generally not used for wood adhesives.
The
advantage of aminoresin adhesives (or amino plastic adhesives as they
are often
called) are their (1) initial water solubility (this renders them
eminently
suitable for bulk and relatively inexpensive production), (2) hardness,
(3)
nonflammability, (4) good thermal properties, (5) absence of color in
cured
polymers, and (6) easy adaptability to a variety of curing conditions.
Although many amidic and
aminic compounds have been
investigated for use in production of aminoresins, only urea and
melamine and,
in rare cases aniline, are extensively used. Thermosetting aminoplastic
resins
produced from urea and melamine are built up by condensation
polymerization.
Urea and melamine are reacted with formaldehyde, which results in the
formation
of additional products, such methylol compounds. Further reaction and
the
concurrent elimination of water, leads to the formation of low
molecular weight
condensates which are still soluble. Higher molecular weight products,
which
are insoluble and infusible, are obtained by further condensing the low
molecular weight condensates.
Urea
and
melamine formaldehyde (UF and MF) resins have a great deal in common as
regards
the chemical and physical characteristics of both the cured and uncured
resins.
MF is superior to UF because of its superior water and heat resistance,
hardness, and shorter curing time under less drastic conditions. The
greatest disadvantage
of these aminoplastic resins is their bond deterioration, caused by
water and
moisture. This is due to the hydrolysis of the aminoplastic or amino
methylenic
bond, which is the same for both UF and MF resins.
The higher resistance of MF
resins to water attack
is due to the considerably lower solubility of melamine in water.
(Melamine
dissolves in hot water only, whereas urea dissolves in cold water as
well.)
Therefore, UF adhesives are used for interior application only MF or
melamineurea formaldehyde (MUF) resins can be employed successfully
even for
rather severe outdoor conditions. If full exterior grade quality is
needed, it
is safer to use phenolic type resins rather than aminoplastic resins.
Chemistry of Aminoresins
A. Urea Formaldehyde Condensation
The reaction between urea and
formaldehyde is very
complex. The combination of these two chemical compounds results in
both linear
and branched polymers, as well as tridimensional networks, in the cured
resin.
This is due to a functionality of 4 in urea (due to the presence of
four
replaceable hydrogen atoms), and a functionality of 2 in formaldehyde.
The most
important factors determining the properties of the reaction products
are (1)
the relative molar proportion of urea and formaldehyde, (2) the
reaction
temperature, and (3) the various pH values at which the condensation
takes
place. These factors influence the rate of increase of the molecular
weight of
the resin. Therefore, the characteristics of the reaction products
differ
considerably when lower and higher condensation stages are compared
especially
solubility, viscosity, water retention, and rate of curing of the
adhesive.
These all depend to a large extent on molecular weights.
The reaction between urea and
formaldehyde is
divided into two stages. The first is the alkaline condensation to form
mono, di,
and trimethylolureas. (Tetramethylolurea has never been isolated.) The
second
stage is the acid condensation of the methylolureas, first to soluble
and then
to insoluble cross linked resins. On the alkaline side, the reaction of
urea
and formaldehyde at room temperature leads to the formation of
methylolureas.
When condensed, they form methylene ether links between the urea
molecules. The
products from urea and formaldehyde, and from mono and dimethylolureas,
are as
follows
The reaction also produces
cyclic derivatives uron,
monomethyloluron, and dimethyloluron.
In weak alkaline solutions,
the first product of the
reaction is a complex (II), which is capable of rearranging itself
exothermically
into monomethylolurea. On acidification, the complex (II) eliminates
water,
resulting in unstable trimethylene urea hydrate (IV). The hydrated
azomethin
(III), which was identified by Fahrenhorst in urea formaldehyde resins,
is
regarded as characteristic of the intermediate stage of the reaction.
The electron theory provides
a possible bonding
mechanism between azomethin groups and either the solvent or other
resin
molecules. In this bonding, theelectrons of the C=N bonds and the free
electron
pair on the nitrogen atoms are involved. The higher pH stabilizes the
degree of
polymerization or association by permitting the formation of ionic
complexes
with water or the solvent. The lower pH causes the loosening of the
water from
the hydrated azomethin groups, allowing association. The resin
eventually
proceeds to the liophobic stage.
Indirect evidence strongly
points to the existence
of this mechanism as well as of the mechanism proposed by the classic
theory of
UF resin formation. However, no conclusive evidence of the
participation of
structure IV in the UF resinification process has yet been obtained.
The
association through azomethine type intermediates has been mentioned to
explain
resin formation and to interpret the mechanism of etherification of
methylol
groups under acid, neutral, and alkaline conditions. This theory
opposes the
classic theory.
In the first reaction,
monomeric methyleneurea is
formed as a result of the intramolecular loss of water. An unsaturated
azomethine group is formed, followed by rapid polymerization. This
gives the
insoluble end product. The other reactions are condensation
polymerizations in
which the methylolureas are merely the building blocks of the polymers
and of
the insoluble end product. The polymers formed in both cases are mainly
linear
polymers obtained by the intermolecular splitting off of water. Under
certain
conditions water may also be split off intra molecularly. Cyclic
compounds
called urones are then formed. In both cases, further splitting off of
water
and formaldehyde leads to the formation of hardened or cured resins.
B. Melamine Formaldehyde Condensation
The condensation reaction of
melamine (V) with
formaldehyde is similar to the reaction of formaldehyde with urea.
Formaldehyde
first attacks the amino groups of melamine, forming methylol compounds.
Formaldehyde addition to
melamine occurs more easily
and completely than to urea. The amino group in melamine accepts easily
up to
two molecules of formaldehyde. Thus up to six molecules of formaldehyde
are
attached to a molecule of melamine. The methylolation step leads to a
series of
methylol compounds with two to six methylol groups.
Because melamine is less
soluble in water than urea,
the hydrophilic stage proceeds more rapidly in MF resin formation than
in UF condensations.
Therefore, hydrophobic intermediates of the MF condensation appear
early in the
reaction. Another important difference between MF and UF is that the MF
condensation and curing occurs not only under acid conditions, but also
under
neutral or even slightly alkaline conditions.
The mechanism of the further
reaction of
methylolmelamines to form hydrophobic intermediates is the same as for
UF
resins, with splitting off of water and formaldehyde. Methylene and
ether
bridges are formed and the molecular size of the resin rapidly
increases. These
intermediate condensation products constitute the large bulk of the
commercial
MF resins. The final curing process transforms the intermediates to the
desired
MF insoluble and infusible resins through the reaction of amino and
methylol
groups which are still available for reaction.
A simplified schematic
formula of cured MF resin has
been given by Koehler and Fry . They emphasize the presence of many
ether
bridges besides unreacted methylol groups, and the methylene bridges.
This is
because in curing MF resins at temperatures of up to 100°C, no
substantial
amounts of formaldehyde are liberated. Only small quantities are
liberated
during curing up to 150°C. However, UF resins curing under the same
conditions
liberate a great deal of formaldehyde.
Wohnsieldler, Updegraff, and
Hunt have tried to
correlate the best physical properties of melamine formaldehyde resins
with the
degree of curing or condensation. They have found that the various
properties
attain their peak at different degrees of reaction. However, the best
physical
properties of MF resins were always associated with significant cross
linking.
C. Aniline Formaldehyde Condensation
When aniline and formaldehyde
are reacted in equal
amounts, under neutral or alkaline conditions, N methylolanilines are
formed.
These form rapidly N methyleneanilines by eliminating water. If heated
further,
they form soluble and fusible aniline formaldehyde resins (VI).
In the presence of acids,
especially hydrochloric
acid, the amino group is protected by the formation of aniline
hydrochloride.
In this case the formaldehyde attacks the free paraposition on the
aromatic
ring. With aniline formaldehyde molar ratio of 11 the hydrochloride of
the p
aminobenzylalcohol is formed. The free base is produced by neutralizing
the
solution of p aminobenzylalcohol hydrochloride. Water is split off and
long
linear chains of brittle aniline formaldehyde resins (VII) of low
mechanical
strength are formed by condensation polymerization.
By using an excess of
formaldehyde over the
equimolar amounts, the linear chains of this product are cross linked,
with the
formation of methylene bridges. The resins produced have a three
dimensional
network of good mechanical strength. In these resins both
aminomethylene
linkages (CH2 NH ) and methylene linkages ( CH2) between the aromatic
nuclei
are present. This indicates that the formaldehyde is able to attack
both the
para and ortho positions of the aromatic rings (exactly as in phenolic
resins),
as well as the aromatic amino group (as in aminoplastic resins).
Amino groups are able to
induce nucleophilicity in
aromatic nuclei, higher than the hydroxy groups of phenols. Therefore,
aniline
and some of its derivatives (m hydroxyaniline and phenylendiamine) are
sometimes
used as terminal grafted groups of linear phenolic resins to accelerate
curing.
In these resins, the aromatic rings and the amino groups form bonds
generally
in a 75 8025 20 molar proportion, respectively. This gives rise to both
aminomethylene ( CH2 NH ) and methylene ( CH2 ) linkages.
D. Reaction Kinetics Urea Formaldehyde
The kinetics of the formation
and condensation of
mono and
dimethylolureas and of simple
urea formaldehyde condensation products has been studied extensively.
The
formation of monomethylolurea in weak acid or alkaline aqueous
solutions is
characterized by an initial fast phase followed by a slow bimolecular
reaction.
The reaction is reversible. The formation of methylolurea is
bimolecular and
its dissociation monomolecular. The rate of reaction varies according
to the pH
with a minimum rate of reaction in the pH range 5 8 for a molar ratio
of 11 for
urea/formaldehyde and a pH of ± 6.5 for a 1 2 molar ratio.
The 12 urea/formaldehyde
reaction has been proved to
be three times slower than the 11 molar ratio reaction. The dehydration
of
formaldehyde (present largely as methylene glycol) and the formation of
the
urea anion are considered to be the controlling factors.
The rapid initial addition
reaction of urea and
formaldehyde is followed by a slower condensation, which results in the
formation of polymers. The rate of the condensation of urea with
monomethylolurea to form methylenebisurea (or UF dimers) is also pH
dependent.
It decreases esponentially from a pH of 2 3 to neutral pH value. No
condensation occurs at alkaline pH values. The marked influence of the
pH range
on the reactions rates indicates that such reactions are of the
hydrogen ion
catalyzed type.
The initial addition of
formaldehyde to urea in
dilute solutions (0.1 M) is reversible, and is subject to general acid
and base
catalysis. The forward bimolecular reaction has an activation energy of
13
kcal/mol. The reverse unimolecular reaction has an activation energy of
19
kcal/ mol. The proposed mechanism of the acid catalysis is that of a
protonated
formaldehyde carbocation with urea. The alkaline catalyzed reaction
proceeds
instead through the reaction of the urea anion with formaldehyde. The
subsequent reaction of monomethylolurea with formaldehyde in dilute
solution, to
give dimethylolurea, corresponds closely to the 11 monomethylolurea
formation
reaction in type, reaction mechanism, and activation energies
It is also reversible in
concentrated solutions (2 4
M) at pH 7.0, and at 35°C, the addition reactions have the same rate
constants
as in dilute solutions and the reactions are very similar. No
trimethylolurea
is detectable in the reactions of urea and formaldehyde in dilute
solutions
containing a 6 8 M excess of formaldehyde . The rates of introduction
into the
urea molecule of one, two, and three methylol groups have been
estimated to
have the ratio 931, respectively. The formations of NN dimethylolurea
and of
trimethylolurea are also bi molecular, and their decomposition
monomolecular.
The formation of N,N dimethylolurea from monomethylolurea is about 1. 5
times
that of monomethylolurea from urea. The decomposition of N, N
dimethylolurea to
monomethylolurea is three times that of monomethylolurea to urea.
No reaction was found between
two molecules of
dimethylolurea under the conditions chosen. It appears that (1) the
amide group
in urea is more reactive than that of monomethylolurea and (2) the
methylol
group in the latter is more reactive than in dimethylolurea. The
results cannot
be interpreted in terms of either dimethylene ether formation between
urea
molecules, or dehydration of methylolureas to methyleneureas followed
by
polymerization. However, an analysis done by the same author s of the
insoluble
fractions of urea formalde hyde condensates indicates that the
properties of
the condensates are the same as those produced by stepwise condensation
to form
methylene bridges between urea residues.
Methylenebisurea undergoes
further condensation with
formaldehyde and monomethylolurea , behaving like urea. The capability
of
methylenebisurea to hydrolize to urea and methylolurea in weak acid
solutions
(pH 3 5) indicates the reversibility of the methylene link and its
lability in
weak acid moisture. It explains the slow release of formaldehyde over a
long
time in particleboard and other wood products manufactured with UF
resins.
Equilibrium constants for
urea formaldehyde
methylolureas have been theoretically derived, and found to agree with
experimental results. They can be used to derive formulas, thereby
quantifying
these components in solution of different initial formaldehyde
concentrations.
Study of the addition and condensation reactions of urea and
formaldehyde have
led some authors to conclude that the addition process is a continuous
one,
with the hydrogen ion acting as a negative catalyst by addition to the
nitrogen
atom of the urea molecule. The condensation, on the other hand, reaches
a state
of equilibrium. In both reactions, the hydroxyl ion can be regarded as
an
indirect catalyst.
A few studies have been done
on the pH changes in
urea formaldehyde solutions during reactions. The main cause of the
observed pH
drop was the Cannizzaro reaction, disproportioning formaldehyde to
methanol and
formic acid. Oxidation by atmospheric oxygen also contributed. The
latter could
be eliminated by working in an inert atmosphere. This, or the use as a
surface
layer of a solvent which will not mix with water, inhibits the
formation of
methylene ureas. Using ammonia as neutralizing agent, an initial rise
in the pH
of the reaction mixture has been observed. This has been attributed to
the
decomposition of unstable methylolamines. The effect of methanol on the
reactions has also been investigated. It slightly retards the addition
reactions. Methanol hinders the formation of methylene links in the
condensation reactions.
E. Reaction Kinetics Melamine Formaldehyde
Melamine and formaldehyde
react in similar way to
urea and formaldehyde, although basic differences are evident in the
reaction
rates and mechanism. The primary products of reaction are
methylolmelamines,
and evidence indicates that such compounds are formed only at ambient
or higher
temperature, except in acid pH ranges. The reaction is reversible
throughout
the pH range. Its forward rate is proportional to either [melamine]
[HCHO], or
[melamine] [H+CHOH] or [melamine] [HCHO], according to the pH of the
media.
Phenolic Resin Wood Adhesive
Introduction
Two
main classes of adhesives
aminoplastic and polyphenolic adhesives. The polyphenolic adhesives are
the
more significant for the production of weather and boil proof wood
products.
As early as the nineteenth
century, it was known
that resinous materials could readily be formed by coreacting phenols
and
aldehydes. From the beginning of this century, much effort has been
directed
toward their commercial exploitation. Baekeland showed in 1907 that the
simplest starting material, phenol and formaldehyde, form products of
commercial importance under the correct reaction conditions.
Phenolic resins thus became
the first true synthetic
polymers ever to be developed. It may have been expected that by now,
their
chemical and physical structure would have been completely elucidated,
but even
now their structure is far from being completely clear. The polymers
derived
from the reaction of phenols and formaldehyde differ in one important
aspect
from other polycondensation products polyfunctional phenols can form a
variety
of isomerides with different chain lengths. Other products derived from
polycondensation reactions, such as polyamides and polyesters, are
mixtures of
molecular chains of various lengths. However, in this case only one
structure
is possible for a molecule of a certain chain length.
For this reason, kinetic and
related studies are
feasible if one assumes that the growth of a chain proceeds from one
molecule
to the next in a smooth and regular manner. Polyfunctional phenols may
react
with formaldehyde in both the ortho and para positions to the hydroxyl
group.
This means that the condensation products exist as numerous positional
isomerides for any chain length. This particularly not only makes
kinetic
studies extremely difficult, but also makes the organic chemistry of
the
reaction very complex and tedious to unravel.
The result has been that
although phenolic resins
developed commercially as early as 1908, and the first completely
synthetic resin
ever to be developed, their chemistry is still only partially
understood. It
may be argued with some justification that such a state of affairs is
immaterial, because satisfactory resins for many uses have been
developed on
purely empirical grounds during the last 70 years. However, it cannot
be denied
that gradual understanding of the chemical structure and mechanism of
reaction
of these resins has helped considerably in introducing commercial
phenolic
resins which have been designed for certain applications, and which are
capable
of performances undreamed of in formulations developed by the empirical
rather
than the scientific approach. Knowledge of phenolic resin chemistry,
structure,
characteristic reactions, and kinetic behavior remains an invaluable
asset to
the adhesive formulator in designing resins with specific physical
properties.
Chemistry of Phenol
Formaldehyde
Condensations
A. Reaction
Mechanisms
Phenols condense initially
with formaldehyde in the
presence of either acid or alkali to form a mehtylolphenol or phenolic
alcohol,
and then dimethylolphenol. The initial attack may be at the 2, 4, or 6
position. The second stage of the reaction involves the reaction of the
methylol
groups with other available phenol or methylolphenol, leading first to
the
formation of linear polymers and then to the formation of hard cured,
highly
branched structures.
Novolak resins are obtained
with acid catalysis,
with a deficiency of formaldehyde. A novolak resin has no reactive
methylol
groups in its molecules, and therefore is incapable of condensing with
other
novolak molecules on heating without hardening agents. To complete
resinification, further formaldehyde is added to cross link the novolak
resin.
Phenolic rings are considerably less active as nucleophilic centres at
an acid
pH, due to hydroxyl and ring protonation.
However, the aldehyde is
activated by protonation,
which compensates for this reduction in potential reactivity. The
protonated
aldehyde is a more effective electrophile.
The substitution reaction
proceeds slowly and
condensation follows as a result of further protonation and the
creation of a
benzylcarbonium ion which acts as a nucleophile.
Resols are obtained as a
result of alkaline
catalysis and an excess of formaldehyde. A resol molecule contains
reactive
methylol groups. Heating causes the reactive resol molecules to
condense to
form large molecules, without the addition of a hardener. The function
of
phenols as nucleophiles is strengthened by the ionization of the
phenol,
without affecting the activity of the aldehyde.
Megson states that reaction 2
(in which resols are
formed by the reaction of quinone methides with methylolphenols or
other
quinone methides) is favored during alkaline catalysis. A carbonium ion
mechanism is, however, more likely to occur. Megson also states that
phenolic
nuclei can be linked not only by simple methylene bridges, but also by
methylene ether bridges. The latter generally revert to methylene
bridges if
heated during curing, with elimination of formaldehyde.
The difference between acid
catalyzed and base
catalyzed processes is (1) in the rate of aldehyde attack on the
phenol, (2) in
the subsequent condensation of the phenolic alcohols, and to some
extent, (3)
in the nature of the condensation reaction. With acid catalysis,
phenolic
alcohol formation is relatively slow. Therefore, this is the step that
determines the rate of the total reaction. The condensation of phenolic
alcohols and phenols forming compounds of the dihydroxydiphenylmethane
type is
instead rapid. The latter are therefore predominant intermediates in
novolak
resins.
Novolaks are mixtures of
isomeric polynuclear
phenols of various chain lengths with an average of five to six
phenolic nuclei
per molecule. They contain no reactive methylol groups and consequently
cross
link and harden to form infusible and insoluble resins only when mixed
with
compounds that can release formaldehyde and form methylene bridges
(such as
paraformaldehyde or hexamethylenetetramine).
In the condensation of
phenols and formaldehyde
using basic catalysts, the initial substitution reaction (i.e., the
formaldehyde attack on the phenol) is faster than the subsequent
condensation
reaction. Consequently, phenolic alcohols are initially the predominant
intermediate compounds. These phenolic alcohols, which contain reactive
methylol groups, condense with either other methylol groups to form
ether
links, or more commonly, with reactive positions in the phenolic ring
(ortho or
para to the hydroxyl group) to form methylene bridges. In both cases
water is
eliminated.
Mildly condensed liquid
resols, important of the two
types of phenolic resins in the formulation of wood adhesives, have an
average
of less than two phenolic nuclei in the molecule. The solid resols
average
three to four phenolic nuclei but with a wider distribution of
molecular size.
Small amounts of simple phenol, phenolic alcohols, formaldehyde, and
water are
also present in resols. Heating or acidification of these resins causes
cross
linking through uncondensed phenolic alcohol groups, and possibly also
through
reaction of formaldehyde liberated by the breakdown of the ether links.
As with novolaks, the
methylolphenols formed
condense with more phenols to form methylene bridged polyphenols. The
latter,
however, quickly react in an alkaline system with more formaldehyde to
produce
methylol derivatives of the polyphenols. In addition to this method of
growth
in molecular size, methylol groups may interreact with one another,
liberating
water and forming dimethylene ether links (CH2 O
CH2 ). This is particularly evident if the
ratio of formaldehyde to phenol is high. The average molecular weight
of the
resins obtained by acid condensation of phenol and formaldehyde
decrease
hyperbolically from over 1000 to 200, with increases of the molar ratio
of
phenol to formaldehyde from 1.251 to 101.
As was expected, the molecule
is nonplanar. The
benzene rings are inclined equally and in opposite directions at angles
of 52°
to the plane of the angle, while the angle is approximately 10° greater
than
the tetrahedral angle. The heat evolved during the phenol formaldehyde
reaction
is 4.1 kcal/mol of formaldehyde added to phenol, and 16.9 kcal/mol of
water is
eliminated during the subsequent condensation. This indicates a total
heat
liberation of some 100 cal/g of anhydrous reactants.
B. Nature of Mechanism Methylene and Methylene Ether Bridges
Lilley
and Osmond separate the
mechanism of condensation of phenol with formaldehyde into two phases.
In the
early stages, when water is present, condensation is almost definitely
ionic.
At a later stage, when the reaction proceeds in unreacted phenol as the
solvent, after dewatering (or in the final stage when reaction is
mainly in molten
polymer), it is almost definitely nonionic.
The reactions are shown as
involving an added
proton, but similar arguments apply by solvolysis with a suitable
ionizing
phenolic medium.
A
methylene bridge is more likely to be formed than an ether link under
acid
conditions and when free positions in uncompletely reacted phenol are
available. This can be shown by considering the behavior of the
concentration
of the carbonium ion when varying the hydrogen ion concentration. In
alkaline
conditions, the concentration of carbonium ions is low, but by
considering the
formation of the ions a similar conclusion can be reached.
It appears, therefore, that
ethers can be formed
only within limited and critical pH ranges, and then only at slow
rates. Thus the
formation of many ether links is unlikely. These critical ranges do not
coincide with technological practice. Lilley points out the need for
research
on the distribution of methylol groups in phenolic resin syrups, and on
the
occurrence of ether links in such resins. All available evidence is
against
their occurrence in large numbers when free nuclear positions are
available, as
opposed to the monoreactive phenol alcohols used as models, where
ethers form
undoubtedly in larger numbers.
Referring to the nonionic
mechanism, which is likely
to occur in the later stages of phenolic resin formation and in the
heat
hardening of phenolic alcohols, Lilley shows that since the reaction
cannot
involve a carbonium ion, it will probably use an intermediate of the
quinone
methide type. This compound is probably generated by a hydrogen bonding
mechanism. This has been shown to occur in phenol alcohols and dibenzyl
ethers.
In the case of a monoalcohol, the reaction will be
A
radical of the quinone methide
type is likely to have a considerably long life. It yields a dibenzyl
ether
easily and a methylene bridge with difficulty. Work done by Van Euler
and
Hultzsch supports the idea that the intermediate appears through
hydrogen
bonding.
This
nonionic mechanism not only
explains the known reactions of phenol monoalcohols, especially as far
as ether
formation is concerned, but also covers fully substituted dialcohols.
The former reaction is much
more likely to occur and
the formation of ethers is improbable. Experimental evidence supports
this
view. Although fully blocked monoalcohols undoubtedly lead to dibenzyl
ethers
in good yield, this is not the case of partially or nonsubstituted
phenol
alcohols. Thus ether forms with great difficulty from saligenin and
homosaligenin and in very poor yield.
Lilley believes that the
formation of dibenzyl
ethers depends on the distribution of activation in the phenol alcohol.
He
suggests that for an alcohol containing a free position, subject to
activation
by the phenolic hydroxyl group, such activation will oppose ether link
formation. Substituents in the benzene ring also affect ether
stability. The
basis for these ideas was obtained by comparing the behavior of two
alcohols, 4
chlorosaligenin and 4 nitrosaligenin.
Each of these has a free
ortho position, and they
are comparable as regards the number of orientation of available
reactive
positions. However, the activations of their ortho positions are
opposite in
polarity. It has been shown that the formation of methylene groups is
bound up
with the d charge effect of the free ortho position. In the nitro
compound, the
ortho position is more positively charged. The lone pair of the
methylol group
provides the most nucleophilic point for the attack of the carbonium
ion, and
therefore ether formation should predominate.
In the chloro derivative, the
negative charge effect
is increased and methylene linkages should predominate. Other factors
must be
taken into consideration. The nitro group directly deactivates the 6
position,
so that reaction there is unlikely. However, the chloro group does not
affect
that position. The nitro group opens positions 3 and 5 for attack, so
that the
chances of methylene bridge formation are increased. Similar effects
were
observed on p cresol. Therefore, the ether is more likely to form in
the case
of nitrosaligenin and less likely to form with chlorosaligenin than at
first
expected. However, results obtained clearly demonstrate that the
proportion of
ether links formed depend a great deal on the activity of the free
ortho
position.
One can conclude that in the
hardening of phenol
alcohols which have free positions for substitution by methylol or its
equivalents, a methylene bridge rather than an ether link will
preferably be
formed . This happens even more for fast reacting phenols such as meta
cresol,
resorcinol, and phloroglucinol, where the charge in ortho position is
considerably increased. It gives rise to a nonionic reaction condition.
Under
ionic conditions, there is a limited range of pH values in which ether
can be
formed. However, this does not correspond with the working conditions
of
generally used pH. The rate of such a formation is slow whatever the
position
of equilibrium. The case of a fully sub
stituted phenol monoalcohol, where ether link formation is
more
probable, is therefore not parallel to that of one with free positions.
This
can be seen by comparing the low yield of ether from 4 chlorosaligenin
with 4,6
dichlorosaligenin (Zinke and Ziegler).
This yields 70% of the
corresponding ether if heated
to 130°C. The use of phenol alcohols as models is therefore of doubtful
validity. Phenolic resins occurring in typical practice are likely to
contain
far fewer ether links than suggested by conclusions drawn on the basis
that
monoreactive monoalcohols are reliable models for study.
Unfortunately, there is little
experimental proof of the
effect of pH on yields of ethers and methylene compounds. Lilley states
that
the reactions of phenolic resins were deduced from those of urea
formaldehyde
resins. The two systems contain identical electronic configurations,
and it
therefore appears justifiable to suggest that a carboniumion mechanism
covers
both cases.
In a later study, Rossouw,
Pizzi, and McGillivray
found that ether links form at pH 4.3 5.0, and that no ether links form
at pH 9
when phenol, resorcinol, and phloroglucinol are used as the model
compounds.
The rate of the formation and decomposition of the ethers and the
amount formed
was observed during kinetic experiments and was found to decrease for
phloroglucinol,
resorcinol, and phenol.
It is important to note that
extremely fast acting
phenols, such as phloroglucinol, do form ether links. (It was thought
that they
do not form ether links because of the instability of their
hydrpxybenzyl
alcohols.) The reason why it is impossible to isolate, for instance,
the
phloroglucinol ether links is because such ethers form and decompose in
the
first half hour of reactions at ambient temperature. However, the ether
links
formed by phenol are much slower to form and to decompose. They are
therefore
stable enough to be isolated and detected.
The range of ether formations
given by Lilley on
deductions derived from aminoplastic resin chemistry are probably
valid. A pH
of 5 is still in the range of ether formations advocated by Lilley
(explain pH
6), and pH 9 9.5 is too far from the pH 8 advocated for their
formation.
Whatever the case, it is certain that ether links form in very narrow
pH
ranges.
C. Acid Catalysis
Consideration must be given
to the possibility of direct
intervention by the catalyst in the reaction. Hydrochloric acid is the
most
interesting case of acid catalyst, and ammonia of an alkaline catalyst.
When
the phenol formaldehyde reaction is catalyzed by hydrochloric acid, two
mechanisms possibly come into operation. Vorozhtov has proposed a
reaction
route which passes through the formation of bischloromethyl ether (Cl CH2 O
CH2 Cl). Ziegler has suggested a route through the
formation of a
chloromethyl alcohol (Cl
CH2 OH) as
intermediate. The second route appears to be the more probable one.
Both
hypotheses agree that chloromethylphenols are the principal
intermediates. The
chloromethylphenols have been prepared and isolated by various means.
They are
highly reactive compounds which, with phenols, form dihydrox
ydiphenylmethanes
and complex methylene linked multiring polyphenols. Reaction is highly
selective and takes place in the para position.
D. Alkaline Catalysis
Different mechanisms of
alkaline catalysis have been
suggested according to the alkali used. In the case where caustic soda
is used
as the catalyst, the type of mechanism, which seems the most likely is
that
which involves the formation of a chelate ring similar to that
suggested by
Price and Sachanandor. The chelating mechanism may initially cause the
formation of a sodium formaldehyde complex or of a formaldehyde sodium
phenate
complex.
When ammonia is used as a
catalyst, the resins
formed are very different in some of their characteristics from other
alkali
catalyzed phenol formaldehyde resins. The reaction mechanism appears to
be
quite different to that of sodium hydroxide catalyzed resins. An
obvious
deduction is that intermediates containing nitrogen are formed. Several
such
intermediates have been isolated from ammonia catalyzed phenol
formaldehyde
reactions by various researchers. Similar types of intermediates are
formed
when amines or hexamethylenetetramine are used instead of ammonia. In
the case
of ammonia, the main intermediates are dihydroxybenzylamines and
trihydroxylbenzylamines.
These intermediates contain
nitrogen and have
polybenzylamine chains.
They react
further with more phenol, causing the splitting
and elimination of the nitrogen as ammonia or amines, and
producing nitrogen
free resins. This, however, requires
a considerable excess of phenol and a high temperature. With phenol
hexamethylenetetramine resins of molar ratio 31, the nitrogen content
of the
resin cannot be reduced to less than 7% when heated at 210°C. When the
rate is
increased to 71, the nitrogen content on heating at 210° can be reduced
to less
than 1%.
Ammonia, amine, and amide
catalyzed phenolic resins
are characterized by greater insolubility in water than that of sodium
hydroxide catalyzed phenolic resins. The more ammonia used, the higher
the
molecular weight and melting point, which are obtained without cross
linking.
This is probably due to the inhibiting effect of the nitrogen carrying
groups
(i.e., CH2
NH CH3 or CH2
NH2) which is caused by their slow rate
of subsequent condensation and loss of ammonia. Ammonia, amines, and
amides
(particularly dimethylformamide) are sometimes used as accelerators
during the
curing of phenolic ad hesives
for wood
products.
E. Metallic Ions Catalysis and Orientation of the Reaction
Bender shows that resins made
under special
conditions, where a high proportion of ortho ortho links are formed in
the
phenol methylene chains, can be cured by hexamethy lenetetramine at a
much
higher rate than resins made with conventional acid or alkali
catalysis. This
involves condensation within the pH range 4 7 by oxides or hydroxides
of
alkaline or alkaline earth metals such as zinc, magnesium, and
aluminium.
Fraser, Hall, and Raum also described the effect of a wide variety of
organic
salts, oxides, and hydroxides of various metals. They came to the
following
conclusions (1) that the orientation effect also occurs with 2, 2
dihydroxydiphenylmethane formaldehyde condensation (2) that the
directive
effect occurs in both the initial condensation and in the subsequent
condensation of phenol alcohols (3) that molecular weight distribution
of high
ortho novolaks differs from those made during normal catalysis (4) that
a molar
excess of phenol in the original condensation is necessary (5) that an
apparent
pH of the reaction mixture between 4 and 7 is needed (6) that the
presence of
electropositive bivalent metallic ions such as Mn+2, Zn+2, Cd+2, Mg+2,
Co+2,
Ca+2, and Ba+2 (in decreasing order of usefulness) is necessary and
that (7)
the order of the efficiency of the metal ions used appears to be
indifferent to
the stability of the unidentified probable complex formed as reaction
intermediate.
Fraser and other researchers
conclude that the high
rate of curing of phenolic resins prepared by metal ions catalysis is
due to
the preferential ortho methylolation and therefore also to the high
proportion
of ortho ortho links in the uncured phenolic resins prepared by metal
ions
catalysis. Normal condensation yields roughly equal quantities of para
para and
ortho para links, but uncured resins produced by metal ions catalysis
yield
approximately one ortho para to two ortho ortho links. They ascribe
faster
curing rates of phenolic resins prepared by metallic ions catalysis to
the
higher proportion of the free higher reactive para positions available
for
further reaction during the curing of the resin.
More recently, Pizzi
explained the mechanism of the
reaction and corrected some of the conclusions which other researchers
deduced
from the work of Fraser. The mechanism presented is similar to that of
the
formation of metallic acetylacetonate complexes. It involves the
formation of
chelate rings between metal, formaldehyde, and phenols similar to that
suggested by price and Sachanan for alkaline phenol formaldehyde
reactions.
Pizzi substantiated the
possibility of such a
mechanism by isolating and characterizing the chromium resorcinol
formaldehyde
and chromium phenol formaldehyde complexes.
The rate of metal exchange in
solution and the
instability of the complex formed determine the accelerating or
inhibiting
effect of the metal in the reaction of phenol with formaldehyde. The
more
stable complex II is, the slower the reaction proceeds to the formation
of
resin III. A completely stable complex II should stop the reaction
proceeding
to resin III. If complex II is not stable, the reaction will proceed to
form
phenol formaldehyde resins of type III. The rate of reaction is
directly
proportional to the instability, or the rate of metal exchange in
solution of
complex II.
As in the case of equivalent
acetylacetonates, the
bonding between metal and ligands in these chelates is partly covalent
and
their stability can be ascribed to their partly aromatic character. The
metal
catalyst does not change its valence, but interacts with molecules or
ions
containing electron donor groups, and accelerates reactions like
hydrogen ions
do. The acid catalysis due to the metal ion differs only in degree from
that of
the hydrogen ion.
The effect of the metal is
stronger than that of
hydrogen ions, because of higher charge and greater covalence, since
its
interaction with donor groups is often much greater. This is important,
because
in the preparation of phenol formaldehyde adhesives for wood, very acid
pH
values cannot be used because they cause wood deterioration. Metal ion
catalysts
have the same catalytic effect on the preparation and setting of the
resins at
a higher pH, which is shown by a higher concentration of hydrogen ions
and
therefore lower pH values.
This allows phenolic resin
adhesives to set in
milder acid conditions, with neither extention of the setting nor any
wood
deterioration. The gel times for formaldehyde obtained for resorcinol
and
catechol indicate that the metal catalysis effect is probably valid for
most
phenols with a free ortho position to the hydroxy group. Most bivalent
metallic
ions accelerate phenol formaldehyde reactions.
The extent of this effect is
directly proportional
to the quantity of metallic ion present. I bivalent metals do not
inhibit the
re action, as the complexes they form are unstable and their rate of
exchange
in solution is high. II they accelerate the reaction in a manner
similar to
that of hydrogen ions, although it is even faster, due to their
stronger charge
and greater covalence. The formation of the identified stable complexes
of type
II and V slows down or inhibits the reaction from proceeding to the
formation
of phenol formaldehyde resins, when trivalent metals are used.
Tannin Based Wood Adhesives
Introduction
The word tannin has been used
loosely to define two
different classes of chemical compounds of mainly phenolic nature
hydrolizable
tannins and condensed tanning. The former, including chestnut,
myrobolans
(Terminalia and Phyllantus tree species), and dividivi (Caesalpina
coraria)
extracts, are mixtures of simple phenols such as pyrogallol and ellagic
acid
and of esters of a sugar, mainly glucose, with gallic and digullic
acids.
They can and have been used,
sucessfully, as partial
substitutes of phenol in the manufacture of phenol formaldehyde resins.
Their
chemical behavior is analogous to that of simple phenols of low
reactivity
toward formaldehyde and their moderate use of phenol substitutes in the
above
mentioned resins does not present difficulties. Their lack of
macromolecular
structure in their natural state, the low level of phenol substitution
they
allow, their low nucleophilicity, and limited worldwide production
somewhat
decrease their chemical and economical interest.
Condensed tannins, on the
other hand, constituting
more that 90% of the total world production of commercial tannins
(350,000 tons
per year), are both chemically and economically more interesting for
the
preparation of adhesives and resins. Condensed tannins and their
flavonoid
precursors are known for their wide distribution in nature and
particularly for
their substantial concentration in the wood and bark of various trees.
These
include various Acacia (wattle of mimosa bark extract), Schinopsis
(quebracho
wood extract), Tsuga (hemlock bark extract), and Rhus (sumach extract)
species,
from which commercial tannin extracts are manufactured, and various
Pinus (pine
bark extract) species not yet commercially exploited. Bark and wood of
trees
were found to be particularly rich sources of condensed tannins;
commercial
development ensued through large scale afforestation and/or industrial
extraction, mainly for use in leather tanning. The production of
tannins for
leather manufacture reached its peak immediately after World War II and
has
since progressively declined. This progressive decline of their
traditional
market coupled with the increased price and decreased availability of
synthetic
phenolic materials due to the advent of the energy crisis stimulated
fundamental and applied research on the use of such tannins as a source
of
condensed phenolics.
Chemistry of condensed Tannins
A. General
Condensed tannins, consisting
of flavonoid units
which have undergone varying degrees of condensation, are invariably
associated
with their immediate precursors (flavan 3 ols, flavan 3,4 diols), other
flavonoid analogs , carbohydrates, and traces of amino and imino acids.
Monoflavonoids and nitrogen containing acids are present in
concentrations
which are too low to influence the chemical and physical
characteristics of the
extract as a whole. However, the simple carbohydrates (hexoses,
pentoses, and
disaccharides) and complex glucuronates (hydrocolloid gums) are often
present
in sufficient quantities to decrease and increase viscosity,
respectively, and
excessive variation in their percentages would alter the physical
properties of
the natural extract independently of contributions related to the
degree of
condensation of the tannins.
B. Monoflavonoids
Monoflavonoids, commonly
known as phenolic
nontannins, represent the most studied group in the commercially
important
tannin extracts because of their relative simplicity. They comprise
flavan 3, 4
diols (leucoanthocyanidins), flavan 3 ols (catechins),
dihydroflavonoids
(flavonols), flavonones, chalcones, and coumaran 3 ones, thus
representing most
of the known classes of flavonoid analogs. Typical are those of black
wattle
(mimosa) bark extract (Acacia mearnsii), where the four possible
combinations
of resorcinol and phloroglucinol (A rings) with catechol and pyrogallol
(B
rings) coexist, although these flavonoids constitute a minor percentage
(3%) of
the total phenolics .
Among the groups of
monoflavonoids noted above, only
flavan 3, 4 diols
and certain flavan 3
ols do appear to participate in tannin formation. Each of the four
combinations
of phenolic substitution is to be found in black wattle tannins. The
main
polyphenolic pattern is represented by flavonoid analogs based on
resorcinol A
rings and pyrogallol B rings (XX). These constitute about 70% of the
tannins.
The secondary but parallel pattern is based on resorcinol A rings and
catechol
B rings (XXI). These tannins represent about 25% of the total bark
tannin
fraction. Superimposed of these two predominant tannin flavonoid
mixtures are
two minor groups of the analogs arising from photosynthetic processes
occurring
in the leaves and immature bark. These are based on phloroglucinol
pyrogallol
(XXII) and phloroglucinol catechol (XXIII) flavonoids. These four
patterns
constiture 65 80% of mimosa bark extract.
The remaining parts of the
wattle bark extract are
the nontannins. They may be subdivided into carbohydrates, hydrocolloid
gums
and amino and imino acid fractions. The carbohydrates pinitol and
sucrose
predominate, with glucose in a lower proportion. The hydrocolloid gums
vary in
concentration from 3 to 6% and contribute significantly to the
viscosity of the
extract in spite of their low concentration. The nitrogen compounds of
wattle
bark extract are mainly the imino acids L pipecolic acid, L 4 hydroxy
trans
pipecolic acid, and L proline, together with lesser quantities of the
amino
acids arginine, alanine, aspartic acid, glutammic acid, and serine. The
nitrogen compounds constitute about 3% of the extract.
Similar flavonoid A and B
ring relationships,
although slightly different and less surely determined, also exist in
quebracho
(Schinopsis lorentzii and balansae) wood extract.
The two main substitution
patterns of wattle extract
are also present in quebracho extract, in which the apparent absence of
the
fiavonols quercetrin and myricetrin and the flavan 3 ols catechin and
gallocatechin is the most obvious and important difference. In brief,
no
phloroglucinolic A ring pattern, or more probably a much lower quantity
of it,
is present in the quebracho extract, as can be deduced by the failure
in
isolating leucocyanidin but in the success of attaining cyanidin
chloride.
Similar patterns to wattle and quebracho are followed by hemlock and
Douglas
fir bark extract. Completely different patterns and relationships do
instead
exist in the case of pine tannins. With the exception of Pinus
ponderosa, whose
principal flavonoid pattern is just about identical to that of wattle
and
quebracho tannins, the other pine species studied (Radiata, Eliotae,
Taeda,
Aleppensi, Sylvestris, Patula, Pinaster, etc.) present instead only two
main
patterns. One pattern is represented by flavonoid analogs based on
phloroglucinol A rings and catechol B rings (XXIII). The other pattern,
present
in much lower proportion, is represented by phloroglucinol A rings and
phenol B
rings (XXIX).
The A rings then possess only
the phloroglucinol
type of structure, with very important consequences in the use of these
tannins
for adhesives. Resorcinol types A rings as well as pyrogallol type B
rings are
completely absent. The main leucoanthocyanidins in these extracts are
leucocyanidin (XXX) and afzelechin (XXIX).
C. Biflavonoids
Of the monoflavonoids
presented, only flavan 3,
4 diols and flavan 3 ola apparently
participate in tannin formation. This is logical, as all the other
flavonoids present
carbonyl groups at the 4 position (etherocyclic rings). These groups
eliminate
the possibility of autocondensation to biflavonoids and higher poly
flavanoids
by both reducing the nucleophilic character of the A rings and
occupying one of
the positions through which natural condensation occurs. Meta
disubstitution or
trisubstitution with hydroxyl or heterocyclic oxygen groups on the A
rings of
flavan 3, 4 diols promote strong nucleophilic centers at the 6 and 8 positions as well as
the formation at the
4 position of benzyl carbonium
ions
stabilized by delocalization of the charge on the vicinal aromatic
ring. Both
conditions enhance the possibility of flavonoid autocondensation.
Consequently,
catechin (IX) and gallocatechin (X) offer the strongest nucleophilic
centers
while leucofise tinidin (V) and leucorobinetidin (VI) provide potential
benzyl
carbonium ions for electrophilic substitution. Logically, attack on the
catechins should be at both of the two available centers (6 and 8 position) on the A
ring and not only on
the 8 position as sustained by a few authors. The proved fact that the
8
position is slightly more reactive than the 6 position does not exclude
the
latter one from the possibility of reaction, as proved by the types of
biflavonoids isolated. Some of the anticipated products based on steric
considerations are in accord with those found among wattle tannins
namely,
leucofisetinidin catechin, leucorobinetidin
catechin,
and leucorobinetidin
gallocatechin.
Both types of biflavonoids,
the 4, 6 and 4, 8 linked
must be present. Similar biflavonoid units which correspond to ( )
leucofisetinidin catechin
have been
isolated from the heartwood of S. balansae (quebracho).
Related biflavonoids were
isolated by Nisi and
Panizzi (from Eucalyptus camaldulensis) . Krishnamoorty and Seshadri
(Mirica
nagi) and delle Monache et al. (Ouratea spp.). Drewes, et al.
characterized the
sterochemistry of two groups of crystalline leucofisetinidins (XXXI
XXXIII)
from the wood of the black wattle tree (Acacia mearnsii) and also
determined
the stereochemistry of a homologous series of all trans
5 deoxyleucoanthocyanidin catechin
from the bark extract of the same
tree (XXXIV XXXVI).
The C
and F
ring heterocyclic systems were shown to have half chain conformations
for most
individual flavonoid moieties, exceptions being twisted boat
conformations
where 2, 3 trans 3, 4 cis relative configurations pertained in the
upper units
(C rings). The x ray structure of 8 bromotetra O methy catechin confirms the 2R, 3S
absolute
configuration of the parent compound catechin.
In the heterocyclic ring both the
heterocyclic oxygen atom and the 4 carbon lie marginally above the mean
plane
of the adjacent aromatic A ring with the 2 carbon and 3 carbon more
definitely
higher and lower, respectively, of the same mean plane. The
conformation of the
etherocyclic ring is midway between a C 2 sofa and a C 2, C 3 half
chair
conformation in the crystal. The phenyl and hydroxy groups at positions
2 and 3
in the heterocyclic ring of the 8 bromoderivative are in the trans
diequatorial
position . Confirmation of the 4,6 coupling for bileucofisetinidins
(XXXI
XXXIII) and the all trans 5
deoxyleuco anthyocyanidins
(XXXIV XXXVI) was obtained by
chemical shift data. The
biflavonoids
(XXXIV XXXVI) from the A. mearnsii bark extract are also accompanied by
their
apparent precursors, leucofisetinidin,
leucorobinetidin, catechin, and gallocatechin and by higher
oligomeric analogs
. Combined, these constitute the tannins of mimosa or wattle extract of
commerce, the mixture having a number of average molecular weight of
1250.
Weinges et al. isolated also 4,8 linked leucocyanidins from various
sources
(XXXVII), while Hemingway and McGraw isolated and identified similarly
4,8
linked biflavonoids from the bark of loblolly pine. The etherocyclic
ring of
both units of these 4, 8 linked biflavonoids were also found to be in
the half
chair conformation.
D. Triflavonoids and Tetraflavonoids Condensed Tannins
Roux et al.
indicated that the principle of condensation based on 4,6 links between
resorcinol units, following initial 4,8 links between resorcinol and
lower
terminal phloroglucinolic units, appear to be a general flavonoid
autocondensation
pattern. While the 4,6 links between resorcinolic units are beyond any
doubt,
considerable doubt still exists about.
1.
The positioning of the
phloroglucinolic flavonoid unit as the tannins lower terminal unit
2.
The 4,8 links being the only
phloroglucinolic condensation pattern possible, as proposed.
This was reputedly shown by
the isolated
triflavonoid condensed tannin from the heartwood of the mopane tree
(Colophospermum mopane) and a tetraflavonoid unit from the karree tree
(Rhus
lancea)
The recurrent 4,6 linkages
between the upper units
was demonstrated by nuclear magnetic resonance (NMR) spectrometry and
by
chemical degradation, while the 4,8 linkage to terminal units is based
on the
interpretation of the shifts of all methoxyl NMR resonances during
progressive
C6D6 addition to CHCl3 solutions of the methyl ethers of the tannins.
The same
author, at different stages, advances two different structures for the
mode of
linkage of the lower terminal phloroglucinolic flavonoid unit.
Structures XXXIX,
4,8 linked, and XL, 4,6 linked, have both been proposed for the same
tetrafluvonoid, indicating the uncertainty of such an interpretation.
Commercially extracted wattle
bark extract (A.
mearnsii) show a similar and continuing system of condensation at the
triflavonoid
level. Descrepancies in the experimental behavior and reactivity toward
formaldehyde of tannin based adhesives, especially wattle tannin
adhesives,
from what expected from any of the
proposed structures XXXIX and XL brought to the
realization that the
phloroglucinolic unit is not available for reaction and should not, in
all
probability, present any free position (nor the 6
or 8 position) available to formaldehyde
attack. If all the structures present should present a lower
phloroglucinolic terminal
unit free to formaldehyde attack, then the initial condensation, on
addition of
formaldehyde, between tannin macro molecules should occur through these
phloroglucinolic units, as phloroglucinolic units are between 10 and 15
times
more reactive toward formaldehyde than the corresponding resorcinolic
flavonoids. Thus a very fast and considerable increase in the viscosity
followed by a somewhat slower rate of viscosity increase of tannin
formaldehyde
mixtures should be noticeable. This is not so and viscosity increase
graphs in
function of time are smooth exponential curves. There are no
indications of
sudden changes of rate of viscosity increase at the beginning of the
curve due
to a change of the species reacting with formaldehyde. Quite
independently from
such applied considerations, Roux and coworkers realized by the use of
high
resolution and progressively high temperature NMR spectra of
triflavonoids
obtained by novel synthetic ways that the most probable structure of
triflavonoids and position of the phloroglucinolic unit are as follows
From structure XLI it is
quite evident that no
position highly reactive with formaldehyde is available on the
phloroglucinolic
unit, as both the 6 and
8 positions are
blocked by other flavonoids. Hence in phloroglucinolic flavonoid units,
all
three 4,6 , 4,8 , and 6,8 linkages are probably allowed. This finding
may bring
to the realization that all the three types of linkages, through
positions 4,
6, and 8 of the phloroglucinolic unit, may be present simultaneously,
with the
formation of branched rather than linear polymeric tannins. It is not
possible
to conclude with the data available at this stage if the tannins
macromolecules
are linear polymers, as assumed up to now, or branched polymers.
A few structural
possibilities are open, such as
1. Branched tannins when
phloroglucinolic units are linked
with the flavonoids at the 4 , 6 , and 8 positions.
2. Linear condensed tannins in
which a polymeric tannin of
type XXXIX or XL has the phloroglucinolic unit no longer terminal but
with the
6 and 8 position,
respectively, blocked
by a further resorcinolic flavonoid unit, which now becomes the tannin
lower
terminal unit,
3. Linear condensed tannins in
which the 6 and 8
positions of the phloroglucinolic
flavonoid unit function as links between two flavonoid chains composed
of 4,6
linked resorcinolic units. This is valid for tannins such as those
present in
commercial wattle and quebracho extracts. As regard pine extracts,
whose
flavonoids are only phloroglucinolic in nature, the accepted continuing
pattern
of condensation is 4,8 linked (XLII),
To conclude, the 8 and 6
positions provide the
nucleophilic function, while the electrophile is presumably represented
by a 4
carbonium ion generated from the flavan 3, 4 diols. In this way the
principle
of self condensation may apparently be continued, resulting in units as
large
as 11 or 20 linked flavonoids, respectively, for wattle tannins and for
polyflavonoids of higher average mass range as quebracho and pine
(average ±
4300).
E. Methods for the Analysis of Phenolic Materials Content in
Tanning
Extract
Various methods of analysis
are available for the
determination of tannincontent. These methods can generally be grouped
into two
broad classes
1. Methods aimed at the
determination of tanning material
content in the extract. These methods were devised to determine which
percentage of the extract would participate in leather tanning. They
may also
be used to give an indication of the amount of phenolic material that
can react
with formaldehyde present in the extract. Their main drawback, as
regard
adhesives, is in the incapability of detecting and determining the
approximate
3% of monoflavonoids, or phenolic non tannins, present in the extract,
which do
not contribute to tanning capacity but which do definitely react with
formaldehyde and contribute to adhesives preparation.
2. Methods aimed at the
determination of phenolic material
present in the extract that can be reacted with formaldehyde. These
methods
were devised particularly for tanning extracts used in adhesives
preparation
and are all based on the dtermination of some of the products of
reaction of
the flavonoids with formaldehyde. Their main defect is that the
phenolic
material content is expressed as an absolute number. They are excellent
in
comparing different extracts as regard their relative phenolic content,
hence
their suitability for adhesive manufacture the absolute numbers
obtained,
though, bears no relationship with the real percentage of phenolic
material in
the extract.
Accepted methods of the first
type comprise the hide
powder method the refractometric method , and various visible,
ultraviolet and
infrared spectrometric methods. Accepted methods of the second type
comprise
the Stiasny/Orth method and its modifications and the Lemme, sodium
bisulfite
back titration method. While an in depth discussion of all the methods
noted
above is beyond the scope of this review, a brief discussion of some of
the
more important or interesting ones is necessary
1.
Hide powder
method. This is the oldest accepted analytical method for the
determination of
tannins in a tanning extract. It is based on the reproduction of the
leather
tanning process in which the material to be tanned is standardized
powdered
hide. The amount of tannins absorbed and fixed by contact of
predetermined
amounts of tanning extract and hide powder is expressed as a percentage
of the
original tanning extract. The method is widely used by leather tanners,
extract
producers, and leather standard and research institutions. All the
extract
producing factories around the world deliver every industrial batch of
tanning
extract they produce with the analyzed percentage of tannins obtained
with the
hide powder method.
2.
Molybdate
ions spectrophotometric methods. These methods are based on the
examination by
visible/ultraviolet (UV) spectrophotometry of the intensely yellow
orange
complexes formed by solutions of molybdate ions with o dihydroxybenzene
and 1,
2, 3 trihydroxybenzene derivatives. Mono and polyflavonoids such as
condensed
tannins, which contain p dihydroxybenzenic (catechol) and 1,2,3
trihydroxybenzenic (pyrogallol) B rings, also form yellow orange
complexes with
molybdate ions in aqueous solutions. An o dihydroxyl group is essential
for
complexalion since five membered chelates are formed, consisting of an
o
dihydroxo group and a central molybdate ion. The absorbance values at
400 nm of
solutions containing an excess of sodium molybdate or ammonium
heptamolybdate
and varying amounts of tannin extract at a buffered pH in the range 4.0
5.6
vary. The amount of condensed tannin present in the extract is obtained
by the
linear relationship existing between tannin amounts and absorbance
values. This
method also has the advantage of being able to detect monoflavonoids,
or
phenolic nontannins, which are overlooked by the hide powder method.
This and
related UV methods are useflul for the determination of the phenolic
material
content of extracts used for adhesives.
3. Infrared spectrometric
determination of A rings. This
method is based on an extension of a method developed by Chow and
Steiner by
using the 1140/1120 cm absorbance ratio, in the infrared region, to
determine
the resorcinol content of phenolic resins. It is based on the linear
relationship between the variation of the infra red 1490/1450 cm 1
absorbance
ratio and the amount of A rings contained in a tanning extract. Of the
two
bands used to calculate the ratio the 1490 cm, one is characteristic of
resoroinolic and phloroglucinolic materials only, while the other is
general of
all the extract components. The ratio of A rings in a wattle tanning
flavonoid
is constant at 33% for resorcinolic flavonoids and 43% for
phloroglucinolic
fiavonoids, it is possible to calculate the percentage of phenolic
material in
the extract. The regression equation for such a system and the limits
of its
application are as follows correlation coefficient r2 = 0.950
y = 0.02375x
0.00275
for 25% × 100%
4. Modified Stiasny method. This
method is based on the
gravimetric determination of the reaction products precipitated during
the
reaction of tannins with formaldehyde in the presence of HCl. The
tanning
extract solution is treated at reflux, in an acid environment, with a
molar
excess of formaldehyde and the fractions of the extractives that are
capable of
reacting with formaldehyde precipitate out of solution they are then
filtered,
dried, and weighed, and the results are reported as a formaldehyde
precipitation number. The method, although not giving the exact
percentage of phenolic
material in the extract, is widely used, as it has the advantage of
giving a
comparative measure of the amount of tannin being capable of reacting
with
formaldehyde under the conditions of formation of a thermosetting
phenolic type
resin.
Reactivity of Tannins as Macromolecules
Considering their
macromolecular nature, condensed
tannins exhibit unique reactions as well as reactions normally expected
of
flavan 3 ols units. Knowledge of the more useful of these reactions is
important in the industrial application of tannin extracts to adhesives.
A. Reactivity and Orientation of Electrophilic Substitutions
of Flavonoids.
The
relative
accessibility and/or reactivity of flavonoid units has been examined by
selective bromination with pyridine hydrobromide per bromide using
units of the
phloroglucinol and resorcinol series. Tetra
O methyl catechin (XLIII) is brominated
preferentially in the 8 position. Only when this position is filled
does
substitution commence at the 6 position.
B. A and B Ring Reactions with Aldehydes and Their Kinetics
Tannins, being phenolic in
nature, undergo the same
well known reaction of phenols with formaldehyde either base or acid
catalyzed,
weakly basic base catalyzed reactions being predominantly used in
industrial
applications. Increasingly alkaline conditions lead to progressive
activation
of the phenol as nucleophile, especially above pH 3, where phenateions
are formed.
The nucleophilic centers on the A rings of any flavonoid unit tend to
be more
reactive than those found on the B ring. This is due to the vicinal
hydroxyl
substituents, which merely cause general activation in the B ring
without any
localized effects as on those found in the A ring. Formaldehyde reacts
with
tannins to produce polymerization through methylene bridge linkages to
reactive
positions of the flavonoid molecules, mainly the A rings. In condensed
tannin
molecules the cap rings of the constituent flavonoid units retain only
one
highly reactive nucleophilic center, the remainder accommodating the
interflavonoid bonds. Resorcinolic A rings (wattle) show reactivity
toward
formaldehyde comparable, though slightly lower, to that of resorcinol.
Phloroglucinolic
A rings (pine) behave instead phloroglucinol. Pyrogallol or catechol B
rings
are by compari son
unreactive, and may
only be activated by anion formation at relatively high pH. Hence, the
B rings
do not participate in the reaction except at high pH values (pH 10)
where the
reactivity toward formaldehyde of the A rings is so high that the
tannin
formaldehyde adhesives prepared have unacceptably short pot lives. In
general,
tannin adhesives practice only the A rings are used to cross link the
network.
However, because of their size and shape, the tannin molecules become
immobile
at a low level of condensation with formaldehyde, so that the available
reactive sites are too far apart for further methylene bridge
formation. The
result is incomplete polymerization that leads to the weakness and
brittleness
that are characteristic of many tannin formaldehyde adhesives. There
are
indications, though, that at least pyrogallol B rings are capable of a
limited
degree of condensation with formaldehyde, even in the presence of an
excess of
the more reactive resorcinol and in mildly acid or alkaline reaction
conditions, as shown by the formation of pyrogallol formaldehyde dimers
and
resorcinol pyrogallol formaldehyde dimers and trimers, with limited
pyrogallol
participation, in model compound reactions carried out at ambient
temperature.
The latter constitutes a rethinking and indicates that notwithstanding
the
possibility of a very limited participation of the B rings to a cross
linked
tannin formaldehyde network, such a network is still weak.
Bridging agents with longer
molecules, such phenolic
and aminoplastic resins, have been used to solve this problem by
helping to
bridge distances too large for interflavonoid methylene bridges. The
latter
trend is evident by the type of industrial tannin adhesive formulations
exposed
in the application part of this chapter. Hillis and Urbach have shown
that
while catechol and the catecholic B rings do not react with
formaldehyde at pH
lower than 10, through their lack of formaldehyde consumption, the
addition of
zinc acetate to the reaction mixture induces the B rings to react with
formaldehyde at lower pH values, the optimum being in the pH range 4. 5
5. 5,
as shown by the higher amount of formaldehyde consumed. This finding
implies
that in the presence of zinc acetate further cross linking of the
tannin
formaldehyde network could be achieved through B ring participation to
the
reaction. This could eliminate the need of increasing cross linking by
addition
of synthetic phenolic and aminoplastic fortifiers. Pizzi has shown that
at the
low levels of addition of zinc acetate which are economical (namely 5
10% on
resin solids), there is an improvement in strength given by a higher
degree of
cross linking, but not enough to give the same performance of fortified
tannin
resins.
As regards the pH dependence
of the reaction with
formaldehyde, it is generally accepted that the reaction rate of wattle
tannins
with formaldehyde is slowest in the pH range 4.0 4. 5 for pine tannins,
between
3.3 and 3.9. The quantity of formaldehyde that reacts with the
polyphenols in
this pH range has been found to be minimal.
At neutral pH, rapid reaction
with formaldehyde at
the 6 and 8
positions of the monomeric
unit occurs, accompanied by a much slower reaction of positions 2 and 6
on the
pyrogallolic or catecholic B ring. The dependence of the gel time of
tannins
with formaldehyde at different pH values is shown in Figure 1.
Formaldehyde is generally the
aldehyde used in the
preparation, setting, and curing of tannin adhesives. It is normally
added to
the tannin extract solution at the required pH both as liquid formalin
solution
and in its polymeric form of paraformaldehyde, which is capable of
fairly rapid
depolymerization under alkaline conditions. Hexamethy lenetetramine
(hexamine)
may also be added to rosins due to its formaldehyde releasing action
under
heat. Hexamine is, however, unstable in acid medium but becomes more
stable
with increased pH values. Hence under alkaline conditions the
liberation of
formaldehyde might not be as rapid and as efficient as described. Under
acid
conditions hexamine decomposes to produce 6 mol of formaldehyde and 4
mol of
ammonia and is soluble in water up to 50% by weight. At an alkaline pH
only 3
mol of formaldehyde is liberated, accompanied by the formation of
trimethylamine. Since, under alkaline conditions, hexamine liberates
formaldehyde only when heated, mixtures of hexamine and wattle solution
exhibit
an indefinite pot life at room temperature. It has been fairly widely
reported
with a few notable exceptions that bonds formed with hexamine as
hardener are
not as boil resistant as those formed by formalin or paraformaldehyde.
This
leads to the notion that the ammonia released is responsible for the
degree of
loss in weather resistance. A few authors have instead conclusively
shown that
hexamine can give bonds as good or better as paraformaldehyde, in
alkaline
environment only this may be ascribed to the formation of
trimethylamine rather
than ammonia with less damage to the bonds formed. The reaction of
formaldehyde
with tannins may be controlled by addition of alcohols to the system.
Under
these circumstances some of the formaldehyde is stabilized by the
formation of
hemiacetals [e.g., CH2(OH) (OCH3) if methanol is used]. When the
adhesive is
cured at an elevated temperature, the alcohol is driven off at a fairly
constant rate and formaldehyde is progressively released from the
hemiacetal.
This ensures that less formaldehyde is volatilized when the reactants
reach curing
temperature, and also that the pot life of the adhesive is extended. In
view of
the fact that the methylene linkages may be too short for the optimum
cross
linking, other aldehydes which also have bifunctional characters have
been
substituted for formaldehyde. Of these, one of the most frequently used
is
furfuraldehyde. This has been found to be unsuitable because of its
slow
reaction with phenols but, pizzi and Scharfetter have shown that
furfuraldehyde
is an efficient cross linking agent and excellent plasticizer for
tannin
adhesives when coupled with formaldehyde. Glutaraldehyde has been shown
to
react with tannins to produce a slow forming precipitate, whereas
precipitates
with formaldehyde forms much faster. The reaction kinetics of
formaldehyde,
acetaldehyde, propionaldehyde, n butyraldehyde, isobutyraldehyde, and
furfuraldehyde with both resorcinolic and phloroglucinolic type
condensed
tannins have been investigated. The same experiments were repeated on
resorcinol, phloroglucinol, and catechol used as simple model compounds.
The tannins have also been found
capable of reactions with
glyoxal and benzaldehyde by means of high temperature gel time
measurements of
tannin solutions with the two aldehydes.
Discreponcies the speed of reaction of resorcinolic and
phloroglucinolic
tannins with various aldehydes have been observed.
Urethane Structural Adhesive Systems
Introduction
A. Historical
Urethane structural adhesive
systems are relative
newcomers to the marketplace. However, in the area of fiberglass
reinforced
plastic (FRP), urethane structural adhesives have captured a very large
share
of the market. This is partly due to the good specific adhesion of
urethanes,
their excellent chemical and environmental resistance, and the high lap
shear
strengths of urethane bonds.
The fiberglass structural
bonding market is a
rapidly growing one. Projections indicate a growth of 15 20%/year for
the next
5 years. Major projects requiring huge amounts of adhesive per part,
for
example, all plastic truck cabs or conventional assembly line parts
with large
numbers of parts (car doors, hoods, or trunk decks), is major growth
areas and
help account for the 15 20% annual growth projections. With this in
mind, let
us proceed to urethane structural adhesive chemistry.
B. Advantages and Limitations
Urethane adhesives offer
significant advantages over
other structural adhesive systems. Urethane polymers can be viewed as a
series
of interconnecting, soft and hard segments, while other structural
adhesives
have one or the other (soft or hard segments), only urethanes have this
unique
combination. The ratio of soft to hard segments may be varied to
produce a wide
range of physical properties.
Urethane structural adhesives
have excellent water
and humidity resistance. The urethane linkage is hydrolytically stable
and
unaffected by a high concentration of water at elevated temperatures.
This
valuable property is important in any bonded part, which will, or
could,
environmental exposure. Urethane structural adhesives are equally
resistant to
salt water and show little or no loss of bond strength when exposed to
salt
spray.
Urethane structural adhesives
can be compounded to
resist high temperature paint bake ovens and to retain structural
integrity
after exposure to 400°F (204°C) for short periods of time. This allows
bonded
parts to be processed on conventional assembly lines.
The disadvantage of two
components, urethane
structural adhesives is that they are moisture sensitive in the uncured
state.
They cannot be conveniently hand mixed, which limits the amount of
material
used to comparatively large quantities. Meter mix machines should be
used for
maximum bond strengths, and their use represents some type of capital
investment. Having machines to meter and mix adhehesive also means that
they
may break down, may go off ratio, and, of course, do require periodic
maintenance.
Chemistry
A. Basic Concepts
Urethanes, as a generic class
of organic chemicals,
are the reaction product of an alkyl or aromatic diisocyanate and a
multifunctional polyol (bi and
trifunctional polyols are the most common, but others are often used
for
special purposes). The reaction is a nucleophilic attack on the
carbonyl of the
isocyanate group by one of the lone electron pairs of oxygen of the
hydroxyl
group of the polyol.
The two most common
diisocyanates are toluene
diisocyanate (TDI) a
mixture of 80% 2,4
isomer and 20% 2,6 isomer and
methylene
bis 4,4 phenyldiisocyanate (MDI). These diisocyanates can be used
separately or
in combination to produce the desired physical properties in the cured
adhesives.
Two types of polyols are
available in the industry,
polyester polyols and polyether polyols. Early urethane polymers
utilized
polyester polyols. Unfortunately, ester linkages are susceptible to
hydrolytic
cleavage, so these early urethanes degraded in a fairly short period of
time,
due to moisture. This, of course, gave urethanes a bad name. At the
present
time, however, the polyether polyols are used exclusively in any
urethanes
which may environmental exposure for example, adhesives.
Diamines are used in urethane
adhesives as chain
extenders. The reaction between diisocyanates and diamines produces
substituted
ureas.
Urea linkages are part of the
hard segment of the
urethane polymer. They are harder than urethane linkages, and this
property can
be used to the urethane chemists advantage. These harder hard segments
lend
better tensile properties and higher heat resistance to the polymer.
Tertiary
amines make good catalysts for the urethane reactions.
The advantages of urethanes
come from their unique
polymeric structure. The combination of hard and soft segments allows
properties of both rigid and elastomeric polymers. The hard segments
provide
good high temperature properties, good tensile strengths, and good
modulus
properties, while the soft segments provide excellent low temperature
properties as well as some elastomeric properties. The combination of
these two
sets of characteristics makes a good structural adhesive.
Application Meter Mix Equipment
Two component urethane
structural adhesives should
be meter mixed to provide consistent, high quality, mixed adhesive. As
the name
meter mix implies, these machines perform two functions
1.
Metering the correct amount of
prepolymer and curative
2.
Mixing the two components to
provide an air free, complete mix.
Several
companies currently
produce such equipment. Some of the requirements of meter mix machines
are
1.
Ability to pump materials of
different viscosities (prepolymer usually of higher viscosity than
curative)
2.
Metering of prepolymer and
curative accurately enough to have a consistently high quality product
(usually
within 5 10%
of the stated ratio)
3.
Temperature controlled
material pots, to ensure consistent gel times year round.
Several
different metering pump
designs are found in the marketplace.
1. Machine utilizes a follower
plate mechanism. This type
of machine is normally used for prepolymers and curatives of high
(75,000
250,000 cP) viscosity. The follower plate type of machine can be used
with lower
viscosity materials, but the cost of this type machine does not warrant
its
use.
2.
Machine is a double action,
air driven piston pump. Cylinders of different diameters are used for
metering
the two components. A recent improvement of the design has been a
shorter shaft
connecting the air cylinder and the pistons, allowing faster cycle
times.
3.
Machine is an impeller type
pump. The impellers, driven by an air motor, drive the pumps, which
meter the
adhesive.
4.
The gear driven type of pump.
This type of machine, unlike impeller machines, has the advantage of no
surge
operation.
All but the follower plate
machines are air
(pneumatically) powered. This is so these machines can be used in
plants where
flammable liquids might be used in production process.
Two different types of mixers
are used in the
industry static and dynamic. The dynamic mixers are grids, inside a
mixing
chamber powered by an air motor. The problem with dynamic mixers is
that the
heat they add to the mixed adhesive makes the gel time shorter. The
dynamic
mixer is, however, easier to clean than the static mixer. Static mixers
are
simple mechanically and do not add much heat to the mixed adhesive.
Cleaning of
small static mixers can be a problem due to urethanes, excellent
solvent resistance.
Burning out of static mixers is not recommended by the manufacturers
but is
often done by the customer.
Curing, Testing and durability
A. Curing
Two component urethane
structural adhesives have the
advantage of room temperature curing. Some customers do use heated
fixtures for
their parts to accelerate the gel time of the adhesive. Obviously, in
high
speed production operations, the ability to speed up the cure is
desirable.
To ensure adequate contact of
adherends, and in the
case of heated fixtures, to accelerate the cure. Unheoted fixtures
should exert
2 5 psi (15 35 kPa) throughout the bondline heated fixtures must exert
20 40
psi (140 280 KPa). The reason for this difference is that at elevated
temperatures [200 225°F (93 107°C)], water preferentially reacts with
the
isocyannte, producing CO2. This CO2 will form bubbles in the cured
adhesive,
thereby weakening the bond. A pressure of 20 40 psi (140 280 kPa) will
force
the carbon dioxide into solution and prevent the bubbling. Temperatures
over
250°F (121°C) will blow the bond due to the same water/isocyanate
reaction. At
these temperatures, however, not enough force can be applied to the
part to
prevent blowing the bond.
A new one component
structural adhesive requires
heat for curing. A time strength graph for several temperatures is
shown in
Fig. Generally speaking, 5 min at 200°F (93°C) or 1 min at 300° F
(l49°C) will
cure the material totally. The one component adhesive has the obvious
advantage
of not requiring meter mix equipment.
B. Testing and Durability
Extensive testing of the
urethane structural
adhesive systems has been done in cooperation with the major automotive
and
truck companies, both in the United States and in Europe. A summary of
the
results of this testing is found in Tables 1 and 2.
Table 1 contains tensile,
percent elongation, Youngs
modulus, 100% modulus, and shore hardness data for six types of
structural
adhesives over a temperature range from
40°F ( 40°C) to 250°F (121°C). The first column represents
a standard,
two component, urethane structural adhesive the second, a version
resistant to
high heat [400°F (204°C) for 60 min] the third, a sandable, paintablc,
two
component urethane the fourth, a two component, urethane elastomer for
use in
the air and oil filter industries the fifth, a one component, urethane,
elastomeric adhesive/sealant the sixth, a new, experimental, one
component
structural adhesive.
All of the data in Table 1
were obtained from primed
0.060 in. (0.15 cm) thick steel. The 1 in. (2.5 cm) overlap bonds were
pulled
in tensile. All failures were cohesive within the adhesive.
Urethane structural adhesives
do adhere to unprimed
metals however; to protect the metal surface a primer is highly
recommended.
Table 2 shows the effect of
substrate and various
environmental conditions on the flex fatigue strength of urethane
adhesive.
Sheet molding compounds (SMC), high glass sheet molding compound
(HGSMC),
directional glass sheet molding compound, cold rolled steel (CRS), and
aluminum
were used as substrates. The flex fatigue test is an Owens Corning
Fiberglass
test for durability. The test consists of placing a bonded panel in the
flex
machine. One end is fixed, while the opposite end is flexed 7.5° to
either side
of the normal plane.
Urethanes also exhibit a
useful gap filling trait
when compounded as adhesives. Table 3 and Fig. 6 illustrate this
property of
gap filling. Often with plastic production parts tolerances are ± 0.020
in.
(0.04 cm). If an adhesive works well only with thin bondlines or with
very
consistent bondlines, it may not be suitable for production parts.
Figure 6 and
Table 3 illustrate that even at 0.110 in. the joint retains a lap
shear,
strength of 1300 psi (9000 kPa), which is enough to de laminate most
SMC and
HGSMC laminates.
Modified Acrylic Structural Adhesives
Introduction
The most recent, and perhaps
the most versatile,
generic family of structural adhesive products to be introduced to the
parts
assembly industry has been designated modified acrylic structural
adhesives.
Additional material pertaining to modified acrylic structural adhesives
may be
found in 1977.
History
Although chemical fastening,
or adhesive bonding, is
as old as recorded history itself, its use has been limited by a
multitude of
factors. Until only recently the use of adhesives had been essentially
a last
resort when no other method of joining could be used from the ancient
Egyptians
manufacturing paper from papyrus by bonding cross plies of fibers to
the
assembly of lightweight, durable, aerodynamically designed aircraft of
modern
day.
With the advent of synthetic
polymer chemistry, the
adhesive technologist in the early twentieth century could begin to
explore
resources other than the naturally occurring raw materials of centuries
past.
The advent of synthetic polymers, ranging from elastomesrs to rigid
plastics,
provided the adhesive technologist with a continuing flow of new raw
materials
phenolics, urethanes, vinyl resins, epoxies, etc.
The earliest, truly
structural adhesives were based
on phenolic resins but were found to be limited by rigidity. They were
subsequently modified with flexible resins, such as polyvinyl buty ral,
to
provide impact strength and flexibility. The limitations of the
phenolic
chemistry mothered the invention of the new epoxy based adhesives of
the late
1940s and early 1950s. The epoxios, until very recently, had been the
state of
the art of high performance structural adhesives. The limitations of
the epoxy
adhesives they require clean, well prepared surfaces they require heat
cures
for best performance they provide only moderate adhesion to the newer
engineering plastics now replacing metals in many areas mothered the
invention
of the modified acrylic structural adhesives.
In the late 1960s and early
1970s, the adhesive
technologists began to exploit the potential of synthetic polymer
chemistry in
situ or within the bondline. In other words, they began to build the
adhesive
polymer and, at the same time, bond the assembly. Very distinct
advantages of
this concept emerged. Poorly prepared surfaces could be tolerated quite
well.
Adhesion to metals was retained, and far improved adhesion to
engineering
plastics was accomplished. Room temperature curing without physically
mixing
two components was provided. Far improved handling characteristics were
evident
because very low viscosities could be applied while high molecular
weight
crosslinked polymers were formed in the bondline, without volatiles or
solvents
being emitted.
With the new advantages, the
modified acrylic
structural adhesives can now begin to compete successfully in many
areas. They
are considered, not as a last choice when no other method of parts
assembly,
such as mechanical fastening or welding is adequate, but as a first
choice when
new assemblies are being designed. In many cases, they are used as high
quality
alternatives to mechanical fastening on parts that are already designed
and in
production.
Performance Properties
The performance of any
adhesive product in any given
assembly is dependent on joint design, application conditions, load
distribution, environmental conditions, substrate properties and in
service
conditions. The ultimate test of any adhesive performance is the actual
in use
history. Regardless, certain standard testing procedures that aid in
the
selection process have been adopted to communicate somewhat
representative
performance values on test configurations.
Tables 1 4 provide
representative data on the three
families of acrylic adhesives. The advantages of these materials over
all other
room temperature curing structural adhesives in processing and
performance are
quite significant.
A. Advantages
1.
Wide substrate versatility commonly
the same adhesive performs satisfactorily on steel or aluminum as well
as on
engineering plastics.
2.
Unsurpassed hydrolytic
resistance and permanence In various aggressive environments. Acrylic
polymers
are well known for resistance to aggressive environments. They are not
readily
plasticized by moisture and can be highly cross linked in situ during
curing
reaction.
3.
Versatility in processing variables
Room temperature cure, without prior mixing, is possible. Mix in
systems may
also be used. Since the adhesives have very low molecular weights prior
to
curing, low power pumps and metering devices may be used even though
non
sagging rheology may be required, even with these 100% reactive systems.
4.
Excellent price to performance
ratios although they provide improved processing and performance
compared to
most cyanoacrylates, anaerobics, and specialized epoxies and urethanes,
the
modified acrylics are commonly priced significantly lower.
5. Minimal or no surface
preparation on metals and
plastics Commonly, mill finished steel and aluminum may be bonded as
received
with little or no deleterious effects on initial strength or long term
durability.
In some cases improved performance is noted on unprepared, as opposed
to
rigorously prepared, metals.
6.
Structural, or load bearing,
physical properties. The modified acrylic structural adhesives provide
extremely high initial bond strength and long term durability on load
bearing
assemblies. Analogous performance with other generic structural
adhesives
normally requires high temperature curing and rigorously prepared
substrates.
7.
Tolerance for poorly mated
surfaces. Although best performance is realized with 5 10 mil
bondlines,
adequate and acceptable performance is possible with bondlines of 1 125
mils or
thicker if necessary.
B.
Disadvantages
1.
Poor adhesion to most
unprepared, cured elastomers and low energy surface plastics. The
modified
acrylic structural adhesives do not readily cure on untreated, cured
elastomer
surfaces. Adhesion to untreated polyethylene, polypropylene, and
various
fluoropolymers is poor.
2.
Distinctive odor. Commonly, in
poorly ventilated work areas, the distinctive acrylic odor is
unfamiliar and
considered unpleasant by some application personnel.
3.
Mix in systems commonly of
unequal portions. Mix ratios of 201 41 are most commonly used in mix in
systems. One to one systems are available for certain applications but
have limited
storage life and less performance versatility. Tolerance to slightly
off ratio
mixing is good.
4. Flash points. The uncured
adhesive fluids may commonly
have flash points slightly below room temperature. Specially developed
systems
may have flash points above 100°F (38°C).
5.
Limited long term, in use
credibility. Due to relatively recently developed technology, case
histories
beyond 10 yr in use are rare. Newer systems have very limited in use
histories.
C. General Performance
Additional performance
properties, such as
resistance to aggressive environments, fatigue resistance, crack
propagation,
effect of bond line thickness, joint design, etc., have been
characterized on
individual products within the families discussed. The results of such
testing
have indicated a high level of durability compared to presently known
structural adhesives. The few major suppliers of acrylic adhesives can
provide
specific durability data on these materials to aid in the engineering
design of
new bonded assemblies.
Curing Properties
Unlike the urethane or epoxy
adhesives, the modified
acrylic structural adhesives cure by a free radical polymerization
process
rather than by ring opening or condensation polymerization. Therefore,
the time
span between detectable thickening and achievement of full handling
strength of
the acrylic adhesives is very short compared to epoxies or urethanes.
This is
viewed as another distinct process advantage of the modified acrylic
structural
adhesives. A cure profile, such as the one depicted in Fig. 1, provides
an
unusual combination of usable repositioning time coupled with a
relatively
short interval between starting to cure and reaching full handling
strength. In
the case of epoxy adhesives, the curing adhesive thickens to a point
where repositioning
is not recommended long before full handling strength has developed. In
the
case of acrylic adhesives, the parts may be repositioned for a period
approaching 80% of the time required to reach full handling strength.
The
caution recommended in the case of the acrylic adhesives is that once
thickening has become obvious, full handling strength will rapidly be
achieved,
so repositioning cannot be tolerated or recommended. With epoxies and
urethanes
some latitude is possible regarding repositioning after the onset of
visible
thickening.
The majority of acrylic
structural adhesives provide
cure times in the range of 2 min to 1 hr at room temperature. In
contrast to
the curing of the epoxies or urethanes, the use of heat to speed the
cure is
not recommended. The cure speed may be accelerated in certain
conditions by
heating at only mild temperatures in the range of 130 150°F (55 66°C).
In no
case should the cure temperature exceed 160°F (72°C) during the initial
stages
of cure. Also, unlike the epoxies, the acrylic adhesives cannot be B
staged.
Once the free radical cure has commenced, it will go to completion by a
self
propagating mechanism.
Technology
The chemistry of the modified
acrylic structural
adhesive is quite complex and extremely versatile, but the chemical
concepts
are rather simple and straightforward. Several patents have been
granted in the
technological area and are beyond the scope of this chapter. It is
important,
however, to describe the concepts in a broad sense.
The adhesive resins are essentially
made up of various
polymers dissolved or dispersed in reactive, unsaturated monomers. This
fluid
also contains free radical initiators, free radical scavengers,
fillers,
nonreactive diluents, and, in some cases, unsaturated oligomers.
Phenolic Adhesives and Modifiers
Introduction
In the early 1900s, Leo
Baekeland discovered a way
to use, for practical a very simple chemical reaction between phenol
and
formaldehyde. The chemistry of this application had been investigated
many
years before. The initial reaction products are a series of relatively
low
molecular weight oligomers with molecular weights from a few hundred to
a few
thousand. With additional heat, and sometimes a catalyst or hardener,
these
oligormers will chain extend and crosslink to yield a phenolic,
thermoset
product.
Baekeland discovered a method
to make useful
products from a resin, which previously had had no special utility.
However, it
was only after 5 years of intensive effort and after many failures,
that he
succeeded in the development of a useful material called Bakelite.
Leo Baekeland had a varied
educational and an
interesting occupational career prior to his discovery of Bakelite. His
early
studies were in Belgium at the University of Ghent. Later he studied at
the University
of London, Oxford University, and the University of Edinburgh. After
coming to
the United States he worked in the area of photographic materials. It
was in
this area that he made a major development, which, after some hard
business
times, gave him the financial freedom to continue investigations into
new
areas.
After a long and systematic
investigation in which
Baekeland tried to study all factors of the reaction between
formaldehyde and
phenol, he found that the reaction could be dissected or separated into
different steps. He also found that pressure was valuable in
controlling the
reaction, and that in the presence of ammonia or another, base he could
spread
the reaction over a longer period and so could stop it at any stage he
wished
by cooling. In 1910 the General Bakelite Company was founded. At this
time
there were a number of industries, which were in need of this Bakelite
material, a plastic, which could be used to mass produce standardized,
interchangeable parts.
The product had excellent
dimensional stability and
electrical properties thus, one of the early uses of the product was an
electrical application. Subsequent to this, the phenolic material was
used in
the automotive industry because of its dielectric strength and its
immunity
from temperature, acids, oils, and moisture.
As early as 1912 there were
hundreds of uses and
applications for Bakelite materials. These included many for the
automotive
related area, such as molded parts, moisture resistant cements, and
timing
gears. However, until 1915 most of the uses were confined to lighting
and
ignition equipment. By 1918 these phenolic based products were also
used as
radiator caps, gearshift knobs, battery terminals, door latch handles,
sliding
circuit connectors, commutators, as well as in spark plugs, gauges, and
as
cement for bonding electric headlight lamp bases. By 1935 the uses had
expanded
further to instrument panels, steering wheels, magnetic couplings,
ignition
locks, robe rails, door lock buttons, ash trays, heaters, and parts of
the auto
radio. Most of these applications were based on molding materials and
on
coating resins (e.g., varnishes). In the early 1930s phenolic resins as
adhesives began to become more important. The typical glues available
at that
time had limitations which resulted in an inability to produce a
uniform
product. Staining, lack of moisture resistance, and lack of resistance
to
bacteria and fungi were all problems which were encountered with
adhesives at
that time. About 1931 development of the use of a new phenolic resin
for
plywoods and veneers began. It was recognized that phenolics had an
advantage
of being chemically inert, and thus, were free from attack by fungi and
bacteria. They were unaffected by heat, cold, and moisture, and did not
stain.
Since these early times of
the plastic industry,
many new plastics have been discovered, but phenolics have remained as
lively
and as important as they were in those first formative years.
At the present time phenolic
resins are used as the
major bonding agent, or contribute to bonding, in a variety of
automotive
related application areas including foundry, friction, abrasives,
fiberbonding,
contact adhesives and sealants.
Chemistry of Phenolic Resins
Phenolic resins are
manufactured from phenol and a
large number of substituted phenols through reaction with an aldehyde,
primarily formaldehyde. Some examples of phenols are cresols, bisphenol
A,
resorcinol, p t butylphenol, p phenylphenol, xylenols, cardenol (meta
substituted alkyl phenol from cashew nut shell liquid), and others.
The major chemical route to
the most common
reactant, phenol, is outlined in Fig. 3. Benzene is initially reacted
with
propylene to yield cumene. Cumene is then oxidized to cumene
hydroperoxide,
which, in turn, undergoes an acid catalyzed rearrangement reaction to
yield
phenol and acetone.
There are two basic chemical
types of phenolic
resins, resols and novolacs. They are differentiated by their phenol to
formaldehyde ratio, the type of catalyst used in manufacture, and the
chemical
structure of the resulting resin. These chemical differences are
further
illustrated in Figs. 4 and 5.
A novolac resin is
characterised by having no
reactive methylol groups but having unsubstituted ortho and for para
reactive
sites where a hardener, such as hexamethylenetetramine (hexa) , can
react to
yield a chain extended and, ultimately, crosslinked polymeric system. A
resol
resin, on the other hand, contains not only open reactive sites, but
also
reactive methylol groups. The result is that resols require only heat
to effect
chain extension and crosslinking reactions. The cure of both types of
resins is
dependent on temperature, catalyst type, hardening agents such as hexa,
concentration of catalyst and/or hardening agents, and the type of
phenol and
aldehyde used.
The chemical reactions
involved in the cure of
phenolic resols, for example, include substitution reactions of the
methylol
groups at a reactive site of another phenolic ring, yielding a
methylene
linkage a substitution reaction by a methylol hydroxyl group to yield a
methylene ether linkage.
The chain extension
crosslinking reactions of the
phenolic resol or novolac result in a fully cured system. Many factors
contribute to the degree of this cure, which, in turn, affects the
performance
properties of the ultimate product. Leo Baekeland in 1909 described the
curing
process as going through three phases of reaction.
The first phase results in
the formation of low
molecular weight oligomers and is designated as A stage. At ambient
temperatures the phenolic may be a low to high viscosity liquid, paste,
or
solid. This A stage product is soluble in alcohol, acetone, or similar
polar
solvents and in sodium hydroxide solution. The solid form will melt on
being
heated.
The second phase involves the
formation of an inter
mediate condensation product and is designated as B stage. In this form
the
phenolic is a brittle solid which is slightly harder than a solid in
the A
stage. The B stage resin is now insoluble in all solvents but may swell
in
acetone or similar solvents. Although it will not melt on heating, it
will
soften and can become somewhat thermoplastic like. Further heating will
take it
into the fully cured C stage. In this stage the phenolic is infusible
and
insoluble in all solvents. The cured resin is now resistant to
chemicals,
thermally stable, and a good insulator to heat and electricity.
Analytical Test Methods
A number of different methods
are used in the
laboratory to elucidate the composition and structure of phenolic
resins and
the chemistry of the curing reactions. These methods include infrared
spectroscopy (IR) nuclear magnetic resonance spectroscopy (NMR)
differential
scanning calorimetry (DSC) thermal gravimetric analysis (TGA) gel
permeation
chromatography (GPC) vapor phase chromatography or gas chromatography
(GC) and
dynamic mechanical analysis (DMA), among others. Each of these methods
offers
some unique insight into the chemistry of phenolic resins. These
methods are in
addition to the normal quality control techniques that are commonly
used.
Typical of the latter are plate flow, gel time, viscosity measurement,
and
other tests that are a function of molecular weight, reactivity, and
crosslink
density of the phenolic resin.
Both IR and ultraviolet (uv)
visible absorption
spectroscopy can yield information on structure and functionality of
phenolic
resins, However, these methods require careful interpretation and
appropriate
standards. A more useful analytical technique is proton or 13C NMR.
Most of the
published work on NMR of phenolic resins discusses proton NMR. In
recent years
13C NMR results have become more abundant, and it is predicted that in
the next
few years solid state NMR will be used more frequently.
Proton NMR quite readily
distinguishes the different
types of substituents and ring linkages between the phenol groups.
Examples of
typical NMR spectra for novolacs and resols are shown in Figs. 8 and 9,
respectively. Note that these spectra give information, not only on the
type
and location of substituents on the phenol ring, but also on their
relative
concentrations.
Gel permeation chromatography
is a variation of high
pressure liquid chromatography designed to separate molecules based on
molecular weight differences. Uncured phenolic resins can be easily
separated
into monomers, dimers, trimers , etc. Fig. 10 for a typical resol and
Fig. 11
for a novolac resin. Note that in Fig. 10 a peak is shown for a
methylolated
phenol monomer and is clearly indicative of a resol resin. Depending on
the
analytical equipment and the columns used, even better separation of
the peaks
can be achieved. Thus, a GPC scan can be an excellent way to
characterize a
phenolic resin in terms of resin structure and relative component
ratios, as
well as resin molecular weight distribution.
Differential scanning
calorimetry and TGA are two
methods which measure the response of a phenolic resin to increasing
temperatures. Differential scanning calorimetry is used to obtain
information
on the resin softening point Tg and on its cure characteristics.
Generally, caution
is advised on interpreting the cure information due to complications
arising
from the formation and emission of by products, such as water and
ammonia (from
hexa). To circumvent this problem, a pressure cell is used to prevent
the loss
of such components, thus yielding somewhat better results.
Thermogravimetric analysis
gives an interesting
insight into one of the key properties of phenolics, that is, their
thermal
stability. This technique measures the weight gain or loss of a
material as a
function of increasing temperature. It is a useful monitor of the
uptake of
oxygen and degradation of the phenolic resin. Examples Fig. 12 These
TGA scans
are obtained on cured materials and show that the oxidation of the
methylene
linkages does not occur until about 752°F (400°C).
With the exception of TGA,
most analytical methods
of characterizing phenolic resins involve measurements on phenolic
oligomers,
that is, A stage resins. For this reason, they often do not yield all
the
information required to evaluate the cure characteristics of a given
resin,
A more recent technique, DMA,
is offering promise
for evaluating phenolic resins during simulated cure conditions. One
type of
instrument, the Du Pont Model 880 DMA, measures the ability of a sample
to
transmit an applied frequency. The standard method of analysis involves
scanning a sample at a programmed rate of temperature increase while
simultaneously monitoring frequency. This frequency response is
directly
related to the modulus or stiffness of the sample which, in turn, is
dependent
on the molecular weight/crosslink density at a specific point in time.
Figure
13 illustrates a typical DMA scan of a solid, resol resin. The DMA scan
gives
information on the melting range and relative cure rates and, in a cool
down
mode, gives a modulus temperature profile of the cured resin system.
In addition to montoring the
samples frequency
response, DMA also provides a mechanical loss scan which yields a
unique
measurement of the Tg and the gel temperature based on a mechanical
property of
the resin. One of the major deficiencies of this scanning method is
that there
are three variables (time, temperature, and modulus) in a two
dimensional
monitoring system. Thus, the kinetics of the cure reaction cannot be
quantified. In order to quantify the kinetics of the cure reaction(s),
a
modified technique has been developed which involves monitoring the
frequency
response of the phenolic sample in an isothermal mode. Rate constants
are
obtained as a function of temperature. From these rate constants a
temperature
response factor for the overall reaction can be calculated. This type
of data
can be used to predict the degree of cure of a phenolic resin under a
given set
of reaction conditions for a given time.
Phenolic resins are available
in a wide variety of
forms and can be used in a large number of applications. The methods of
choice
for evaluating a phenolic resin are dependent on the application area.
Many
times simple quality control methods are all that is needed and
required. At other
times a more detailed analysis is necessary in order to compare resin
structure
and properties with actual application performance needs. The following
selected section will discuss phenolic bonding applications in the
automotive
area. A more detailed discussion will be presented on phenolics as
modifiers
for adhesives.
Phenolic Adhesives
Phenolic resins can be
bonding agents as neat resin
(adhesive) or as part of a formulation (phenolic modifier). The
phenolics have
good adhesion to polar substrates, good high temperature properties,
resistance
to burning, and high strength. Phenolics are used as bonding agents in
fiberbonding, friction, abrasives, and foundry applications, among
others, all
of which utilize the material as a neat resin.
In the fiberbonding area the
phenolic resin is used
as a binder in products such as thermal insulation batting, automotive
acoustical padding, and cushioning materials. These products can
consist of a
variety of fibers such as glass, mineral, cotton, and polyester laid
down in a
randomly oriented, loosely packed array to form a mat. The phenolic
resin is
used to bond the individual fibers together using either a dry bonding
or a wet
bonding process. The dry bonding process uses a pulverized phenolic
resin,
either a resol based or a novolac hexa system, to bond reclaimed
fibers. Automotive
acoustic padding, normally involving organic fibers, and low grade
thermal
insulation batting, utilizing glass fibers, is made using the dry
bonding
process. Resol based phenolic resins offer some advantages for
manufacture of
both types of products, especially for the glass based materials, but
this type
of resin requires special handling and refrigerated storage. In the wet
bonding
process virgin spun glass is bonded with a liquid, low molecular
weight, water
miscible resin. Higher grade thermal insulation for construction
applications
is made by this process.
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